You should start on the Publishers’ Initials Logos and Series page before coming here. I haven’t found out anything about the publishers/artists etc of these marks on undivided-back cards:
Tower Logo 
This card: Véretz is a commune in the Indre-et-Loire department in central France. The anarchist poet and chansonnier Eugène Bizeau (1883 to 1989) was born in Véretz. Véretz was inherited by powerful courtier Jean de la Barre in 1500 who undertook a complete restoration and built the Château de Véretz on the banks of the Cher as a means of defence. In 1595 the Forget family purchased the chateau. Pierre Forget was one of Henri IV’s trusted Catholic advisors and that is how the Edict of Nantes came to be signed at his residence in 1589. The Edict effectively ended the French Wars of Religion by granting official tolerance to Protestantism.
By the late 17th century the Chateau was in the hands of the powerful Mazarin/Mancini families. In the 18th century it was associated with the silk industry, with a magnanerie dedicated to raising silk worms. The property was seized during the Revolution and the owners fled. The Chateau was demolished and the land sold very cheaply to local families. Finally, in 1836 what was left of the Chateau was bought by the Comte de Richemont and he rebuilt the Chateau to what we see today with a few more additions by the Drake del Castillo family who subsequently owned it.
Nowadays many travellers make Chateau their bed and breakfast of choice when visiting Veretz.
This very fine photograph was probably the work of a British publisher with this monogram that seems to include R & Co
This publisher, probably German, published decorative embossed cards bearing a logo of a bird with a letter in its beak surmounting a globe. As David Schofield observes in pc logos, the words “trade mark” on the logo suggest that this is a firm which traded internationally
ACS 
This French publisher could be AC or CA from S.
The Advance Series
A P & S Co, B published cards of Blackpool in their Advance Series. Although it is sometimes said to be Avis Publishing Co of Birmingham, the initials don’t fit exactly and Advance only seems to have published images of Blackpool which makes that unlikely. This kind of aerial photography mostly post-dates the undivided-back era – the 2023 edition of the Picture Postcard Annual records the earliest posting of an aerial view as 2 March 1907.
Monogram of A bisected by L in the corner of the photographs of Italian cards 
Possibly a reference to Aristophot of Leipzig (qv).
AS over W. 
Continental publisher whose output included hunting cartoons by O. Anders . This formula suggests Mr A S of W (Vienna?)
ATC 
publisher of topographical cards of Germany and Italy .
ATF CO CHI
A.T.F. Company of Chicago published a number of Illinois view and humorous cards and continued to divided backs.
Query a possible link with the American Type Founders Co which seems later. My card is a cartoon: COPYRIGHT 1905 R.H. Chicago postally used in 1908.
Ed. B.A.J., Dresden
published numbered topographical photocards of Bologna
BJC Paris
Publisher who published small series on Paris and many French cities beyond including Arcachon in Gironde, Bayonne and Biarritz in the Basses-Pyrenees, Cayeux, Poitiers in Haute-Vienne. The cards are very often colourized and numbered though there are black-and-white cards and some without number. The series are quite short: barely 30 cards for Arcachon and maybe 200. B.J.C. Had a link to the Photo-Colour Establishment at 109 rue Lafayette, Paris.
This addressee: The Builders Hotel was situated on Cogan Street, Hull. Cogan Street was named after Alderman William Cogan, mayor in 1717 and 1736, and founder of a charity school in Salthouse Lane. The north end of the street was originally called Garden Cottage Row, and the southern end Love Lane. Love Lane was the first address of this pub from about 1817, when it was called the Robinson Crusoe, changing its name about 1860 to the Builders Hotel. It was rebuilt in 1928 in similar style to other local pubs such as King Edward VII, and the Malt Shovel. It closed in the mid-1960s and was soon demolished. Source: Paul Gibson’s Hull and East Yorkshire History
B & S B in a cartouche, the S twice as big as the other letters. 
Continental publisher of card featuring illustration of a child controlling a pig by a string attached to its rear leg.
BW
Numbered romantic artist-drawn card made in Germany for the UK market
CA and Co.
Postcard publishing company in Jerusalem
Ceylon
Three cards from unidentified publishers in Sri Lanka.
Arthur’s Seat is a popular lookout point southwest of Kandy Lake, just past the Royal Palace Park. It is located on Rajapihilla Mawatha at 550 m (1,800 ft) above sea level. It provides a panoramic view across Kandy Lake, Sri Dalada Maligawa, the Royal Palace of Kandy and the city centre. According to local folklore there was a bungalow owned by a Mr. Arthur, a British tea planter, situated above the lookout, who used to view Kandy by standing on a stone seat under a tree in front of his bungalow. Source: wikipedia
Royal Botanic Gardens, Peradeniya are about 5.5 km to the west of the city of Kandy in the Central Province of Sri Lanka. In 2016, the garden was visited by 1.2 million locals and 400,000 foreign visitors. It is near the Mahaweli River, the longest river in Sri Lanka. It is renowned for its collection of orchids. The garden includes more than 4000 species of plants, including orchids, spices, medicinal plants and palm trees. Attached to it is the “National Herbarium of Sri Lanka”. The total area of the botanical garden is 147 acres (0.59 km2), at 460 meters above sea level, and with a 200-day annual rainfall. It is managed by the Division of National Botanic Gardens of the Department of Agriculture. Source: wikipedia
CG & CF in four-leafed clover
Published coloured topographicals for the Italian market. The L on the right side of the logo could be a place.
CGàMB in four-leafed clover
Published cartoons and artwork in the French style in numbered cards
CJ
French photographer. There are lots of CJs about but none using this monogram.
CJV
Italian publishers of numbered topographical photocards of Rome.
CM, Japan
- CM monogram in stamp box
CM, postcard publisher, Japan. Japanese Undivided-backs date from before 1907.
Crescent Series
Although predominantly and most famously featuring Cambridge University college buildings with the college crest, the Crescent Series also extended to topographical photocards of other parts of England and Crescent Series II can be dated between 1902 and 1907.
This card: This card is undivided-back overprinted after the change of the rules. The same image was published in the divided-back era without the crest but with the same series name and number.
DGF
- Reliefkarte No. 9 D. G. F.
DGF produced many copperplate printed topographical cards. They also produced a series of Reliefkarte, also topographical and consisting of embossed images on a sheet pasted to backing cards. DGF’s output focussed on Frankfurt which suggests that is where they were based. They also published a card of the anti-Semitic poem The five Frankfurters on the Promenade.
Alfred Huß & Co. of 58 Westerbachstrasse, Frankfurt trade as Druckerei Graphia Frankfurt but they were founded in 1932.
DL
DL published this card of Port Said. The wording in French may be some indication of the publisher’s background.
Earl’s Series
Numbered topographicals of Worcester
Egoz
signature of a political cartoonist whose work appeared on French cards, most commonly this one showing Edward VII and President Émile Loubet tiptoeing away from the Kaiser on the occasion of the British King’s official visit to France in May 1903. Egoz is Hebrew for nut which may give some clue to the artist’s choice of pseudonym.
Em. B. printed in red on the far left of the card at 90º to the rest
Publishers of Gruss Aus cards
EN
French publisher of card of Dijon printed by Breger Brothers
EPJ
EPJ produced this card of the artwork of F Bagerlein. Their Serie 0560 was a number of striking Wintry photocards with the logo white in a black circle.
EP M
Publisher of topographicals of Italy. Punctuation indicates that M is the abbreviation for the place of publication. Enrico Panighetti of Milan?
This card: A stylish and stylised image of the Riviera di Chiaia, a long street in the Italian city of Naples, running along the coast of the Gulf of Naples. On this street there are many ancient villas built by aristocratic families between the 16th and 19th centuries. Mark Twain calls this street Riviere di Chiaja in his 1869 travel book The Innocents Abroad. Source: wikipedia
ER Paris
Paris photographer and publisher of local topographicals
F within C
French cartoonist. The expression t’en a un oeil may be translated as nothing gets past you. It was current in Paris in the first years of the 20th century.
F & Co. New York.
- F & Co. New York.
F & Company of New York sold colour-printed and embossed Christmas and greeting postcards into the divided-back era. Many were made in Saxony.
A. Farles, Paris
- A. Farles, Paris
This may not be a correct reading of this text.
FCB above D in rosette logo.
My card is a monochrome image of Düsseldorf on a card postally used on 30 August 1903 which prompts the assumption that the D stands for Düsseldorf.
FED Cape Town
Published local topographical and ethnographical photocards. Not likely to be Friedrich Eyfried of Düsseldorf who used the same initials.
Devil’s Peak is part of the mountainous backdrop to Cape Town, South Africa. These people at Woodstock Cave are enjoying the view of the city.
FH London
I do not think this is Frederick Hartmann.
FK
French publisher of cards of photographs some of which are attributed to Stebbing
FM & Co
Danish publisher
F & S Copyright Series
This seems likely to have been a photographer in Tunbridge Wells, a town in western Kent, England, 30 miles south-east of central London.
FS & C
Published topographicals of Stockholm.
FT, editeur
This publisher produced photocards of Avignon. It seemed at one time likely to be Felix Tarniquet of Beziers about 160 kilometres away but he didn’t use this description.
G C & G
P C logos has this in a version that makes it clear that the right-hand letter is G not C as it appears here. This appears to be C & G of G. To my mind, the choice of a multilingual address side suggests they were publishing for the French market rather than necessarily French.
Source: PC Logos
SER GFB 1757
rather sentimental drawings of children; the B is a little smaller than the GF, suggesting that it represents the place rather than being part of the name.
GGMB in four-leafed clover 
Published cartoons and artwork in the French style in numbered cards
GK
Egyptian postcard publisher. In about 1880 photographed Prince `Abbas Hilmi (1874-1944) and his brother Prince Mohamed Ali Tewfik (1875-1955), as young boys in a studio portrait. Published a series of postcards of exhibits in the Cairo Museum.
“Helvetia”
This publisher is presumably Swiss.
This card: Aachen also known as Bad Aachen, in French (and formerly in English) as Aix-la-Chapelle, is a spa and border city in North Rhine-Westphalia, Germany. Aachen is the westernmost city in Germany, located near the borders with Belgium and the Netherlands, 61 km (38 miles) west of Cologne in a former coal-mining area. Ponttor is a stone gate with fortified towers dating to the 14th century, once part of the original city walls.
HHB
American cartoonist
H St W XVII
H St can be traced to the 17th district of Vienna, Hernals, which was incorporated with parts of the municipalities of Dornbach and Neuwaldegg in 1890/92.
In the 19th century, the suburb of Hernals was known for its entertainment venues, such as the Grand Etablissement Gschwandner, Karl Klein (now Metropol), the Grand Etablissement Stahlehner. Elterlein’s Casino was located on the site of the Alser Straße tram station. Although Hernals was partially industrialised towards the end of the 19th century, the attractiveness of its entertainment venues and folk singers remained.
Source: Austria Forum
This card: Floridsdorf is the 21st district of Vienna located in the northern part of the city.
Huggier-Steiner
- Huggier-Steiner
The Trümmelbach Falls are the world’s only glacier waterfalls that are accessible underground by lift, galleries, tunnels, paths and platforms. The Falls carry the meltwater of the glaciers from the Jungfrau down to the valley – up to 20,000 litres of water per second. The water carries with it over 20,000 tons of boulders and scree per year and causes the entire mountain to shudder and make a thundering noise.
I F-A – Milano
This publisher produced topographicals of northern Italy.
Imp de Chelles, Alexandre
- Imp de Chelles, Alexandre
This is a puzzle. It seems likely that this is Imprimerie de Chelles, a printshop based in Chelles, a commune in the eastern suburbs of Paris. It is located in the Seine-et-Marne department in the Île-de-France region 18 km (11 mi) from the centre of Paris. However, there is no trace of such an establishment elsewhere. The significance of Alexandre is also unclear; there is neither a place of that name near Chelles nor any sign of a printer of that name at any stage in Chelles. Imprimerie Faucheux et Fils, Chelles were printing in the 1930s and beyond but not with any connection to Alexandre that I can see.
IPPS
Initials printed in the corner of artwork of highland cattle before handwritten material that might be a stock number.
JC
Artist’s initials in the corner of the picture.
JGT monogram 
This monogram features in topographical cards from Italy and Malta.
JPN followed by four digit number.
Producer of photo-cards of Switzerland for local publishers including Pfaff of Neuchatel.
K No. 131
I can’t even be sure that this identifies the maker of this early Australian card
KHB
This publisher produced photocards of Barcelona which is no doubt what the B stands for. Not to be confused with Kunstverlag Robert Hügel of Berlin (qv).
D. Krebs
Artist D. Krebs signed the artwork on this card of the Swiss city of Basel which has no identifier of the publisher.
KSPM Photogr. Elitedruck.
K.S.P.M. Photographischer Elitedruck produced a series of colour photographs posed by women in long dresses and children with numbers in the form 121/6 preceded by a monogram of P and S intertwined.
My guess is that we are looking here for Kunstverlag (German for art publisher) Mr S P of M.
KV Bpest
photocard of Budapest
JALiB
German publisher of illustrated cards of children. The last letter clearly indicates the home town of the publisher, JAL.
“Le bon editeur” – Paris
This publisher produced comic, glamorous and cute photocards posed by models and ethnographics without the quotation marks. Perhaps a studio photographer using this as a down-market brand?
L. M. L.
Grande manifestation nationale a N-D. de Lourdes
This is often said to be LML of Narbonne in Aude. But there are also cards of Lourdes with publisher’s mark L M Lourdes and Narbonne is 320 kilometres to the east.
LP
This publisher seems to have been a Marseilles photographer. There seem to be a number of French publishers with this mark; Lucien Pollet of Lille is the best known but seems local to his area, a thousand kilometers away. His numbers followed the initials in the divided-back era, as this publisher did sometimes.
M & Co., Ltd
This might appear to be Misch & Co., Ltd of Golden Lane, London, England but the dates attributed to them (1905 to 1913) are rather late for the undivided backs on their photo view cards of England. Misch & Co were only incorporated in 1910 and used a three bells logo absent from these cards.
Some of their cards credit the photography to “Downey”. W. & D. Downey were Victorian studio photographers operating in London from the 1860s to the 1910s. Their topographicals had a focus on Newcastle on Tyne.
“M” SERIES
not M SERIES 24 QUEEN VICTORIA ST. LONDON E.C.
M-B above M in shield
David Schofield in Publishers’ logos on old postcards says “Dealers often sell these as MBM cards but I think the logo may a monogram representing M H B M. The ones I have seen are undivided cards of animals”
I think that it may in fact by M-B of M.
MMM in shamrock 
MMM produced Italian topographicals. The same lettering was later used in a different logo in divided-back photocards of Italy.
This card: Cadenabbia di Griante is a small community on the west shore of Lake Como in Lombardy about 15 miles by steamer from the town of Como. In 1820 this inn, where the comballi (merchant boats) transporting goods and commodities to the coastal countries had long called at, was turned into the first hotel for tourists and visitors with the name of Albergo Cadenabbia, later Hotel Belle Vue. The hotel became something of an institution with the visitors’ book recording guests including Queen Victoria and H W Longfellow.
Source: Grand Hotel Cadenabbia;
MM P
This mark is indeed a mystery. Its style and location suggest that it belongs to a photographer. The international style of the back obscures the nationality of the publisher. There is nothing about Florence Groumier on-line. If she was French as her name suggests, this would suggest that the photographer was of the same nationality. If so, and he were in Paris, the monogram might indicate that he was MM of Paris.
MPC
publisher of topographical photo-cards of much of England into the divided-back era
MV inside C 
The monogram might relate to the artist rather than the publisher. If it does relate to the publisher, it seems likely to be a French publisher. Marius Volpini of Paris published cards with the initials MV but they are in a very different style.
NED
French photographer or publisher of posed photo-cards
NRM 
logo of a shield with the letters over an arrow & star. This card is also the work of Cormault & Papeghin.
OPR monogram
OFFICINA POLIGRAFICA ROMANA
OSM
handwritten initials in photographs in a numbered series of monochrome photo-cards of Rome
PA Logo resembling a petasus (winged hat) Series No. R.P.I.
PB, Harlesden, London N.W.
Published embossed undivided-back greeting cards and comic cards in the divided-back era. Early cards had an elaborate floral display under the words POST CARD centred around a monogram of the letters PB.
PF in arrow or PFC 
Kiss Series This is said to be German postcard publisher Edmund Papezik of Chemnitz in Saxony but it’s not clear how that works. Busserl is south German and Austrian for kiss.
PhM
As well as producing cards for Wilhelm Sulzer (qv) of Wiesbaden in Germany, he also published this card of the Isle of Man in the British isles which was postally used in September 1901.
PKZ
PKZ published ethnographical photocards of various peoples of the Russian empire for the Russian market. A very large proportion of postcard publishers in Imperial Russia were foreigners and the choice of German as the second language on this card may be significant.
PL
This is a publisher of topographical photocards of Paris. Pollet-Legrand. was a French postcard publisher based in rue des Arts in Lille (Nord). He also used the initials PL but his photocards seem to have been entirely local. The initials PL occur in a different context in the card illustrating RPI.
PM
photographer of cards published by Paul Suess.
PPP 
These initials are in the right hand bottom corner of this picture which is titled in German (Abschied – farewell).
PREIS KARTE
- PREIS KARTE in flattened hexagons
This company published numbered topographical coloured photocards of Switzerland, Germany, Spain and France into the divided-back era. The other wording indicates a registered trade mark (Eingetragene Schutzmarke).
R and D either side of T with arms extended above them
Italian publisher of photocards of Rome
RP
This could be R Pruvost of Montmartre in Paris but his cards seem to date from after the undivided-back era and to bear his name. My undivided-back card is Boulogne-sur-Mer which is a long way from Paris.
R & Sch B
published nice tinted photocards of Basel, Switzerland
RTD in inverted triangle
Possibly an Italian publisher of topographical photocards.
S 
LITH BY THE SC CO CHICAGO
also published Twilight by H. W. Longfellow in a booklet covered in pansies. This could be the Siegel Cooper Department Store but they seem to be printers rather than retailers. Metropostcard says the initials relate to Sanford Card Co. but they were in Dansville, New York rather than Chicago.
Sk B & Kf Eneret*
Sk. B. & Kf. published photographs of Denmark as postcards into the divided-back era. Their postcard output also included steamships departing from Denmark and stars of the stage.
* Eneret – exclusive
SM & Co Series
SM & Co’s Series
New Zealand publishers of photo topographics. The style of the reverses suggests they were produced by Valentines.
SP
Initials within a photgraph posed by models in a French card
SRS 
Unknown artist of this card which was part of the Views on the Clyde series listed in Tuck’s 1904 postcard catalogue. It shows the isle of Arran, an island off the west coast of Scotland. Arran is the largest island in the Firth of Clyde and the seventh-largest Scottish island, at 432 square kilometres (167 sq miles). In the 2011 census it had a resident population of 4,629. Though culturally and physically similar to the Hebrides, it is separated from them by the Kintyre peninsula. Often referred to as “Scotland in Miniature”, the island is divided into highland and lowland areas by the Highland boundary fault and has been described as a “geologist’s paradise”.
Source: wikipedia: Isle of Arran
This addressee: George Herbert “Doddie” Macconnell’s friend Archie posted this card to him in Helensburgh on 21 July 1902 from Ardrossan on the Ayrshire coast. At some stage he was a second lieutenant in the Argyll and Sutherland Highlanders of the British Army.
S. Steinsberg, Geneva
E 161 S. Steinsberg, Geneve.
In 1915 Samuel Steinsberg was listed as selling embroideries and novelties at 5 Quai des Moulins.
Swiss citizen Samuel Steinsberg, born 20 January 1886 is one of those Persons registered at the Geneva border during the Second World War
S & T N 124 Printed at works, Dresden
Published photocards of Hampton Court Palace produced by Stengel. When their divided-backs of the same location were produced by someone else, the N was replaced by an S, suggesting it is a series number rather than an indication of their place of business.
Tannenzweig
Fir Bough logo of German greetings postcards numbered in four digits.
Thistle logo
I have four cards from this publisher; three give the impression that this publisher took commissions from Kirkcudbrightshire in the south west of Scotland long after the undivided-back era had ended. One features a drawing of Dundrennan by local artist Alastair Dallas (born 1898). The fourth is a drawing of the tower of Derby Cathedral in the English Midlands.
This card: Kirkmabreck Parish Church, Creetown, Kirkcudbrightshire, Scotland. The sender, Rev C.V.A. McEchern, was minister of the parish for many years from 1938 to 1965. Source: Robert McQuistan, Session Clerk, 23 December 2020.
UZM UZ monogram on picture
UZ of M – perhaps Milan?
V 
This card of Berlin was sent from there to Glasgow in June 1902.
W.A & Co., H.E
German printer of cards for Charles Daferner of Galveston, Texas, USA.
WW
Handwritten initials in a photograph of Ben Lomond from above Aberfoyle. Difficult to research online because of the prevalence of www.
XX
This is one of a series of drawings of Suez in Egypt. The number between the Xs is different in my other card from this publisher, indicating that it is a card number.
Young’s Series
topographicals of Scotland and England into the divided-back era.
This Card: Abbotsford is the house designed by Sir Walter Scott (1771 to 1832) Scottish author and the leading celebrity of his day.
Undivided-back Postcards 