I am going to try to be as concise as I can be, but this is a very complecated set to deal with.
The Sheet and the advertising from Koester's Bread feature 40 cards, however, I do believe the actual distribution of cards to be 52, as some not found on the sheet have been found in unmistakably period Koseter's accumulations.
A study of the E121 family of sets needs to be done with a "timeline" of production being kept in mind. Obviously, before the 1921 World Series between the two New York teams there were already New York players in both the E121 set and being distributed by anonymous companies as what we would call W575-1's (they chose not to stamp or print ads on the back of their cards). Those cards from early print runs are the ones that are difficult to pin-point being from another W575-1 source or from the Koester's distribution.
About the thickness of cardstock, if they are on very thick stock they should be hand-cut from the poster/sheet (whatever you want to call it). The regular cards distributed in the bread are on stiff stock but are very similar to the e121 Series of 80, in contrast the E121 Series of 120 (and later sets like Lou Gertenrich) are on a distinctly thinner card stock and are actually harder to find in nice shape, even though the series itself is far less popular than it's earlier Series of 80 cousin. Hold one of each card in each hand and you can tell the difference pretty easily.
About the Koester's Bread set... we know that they distributed the series of New York players as we have seen the album and the period advertisiments. That being said, there may or may not have been other companies that also distributed the New York cards but no advertising has been found to support that, but as there were many different distributors over the years of the cards of this format (E121/D383/D350?etc, etc.) I bet the manufacturers of the cards would have entertained any other outfits wanting to distibute a "World Series" set as well. We know that The American Caramel Co. also got into it and distrubuted their own version of the "World Series" cards for insertion in it's E121 set in the versions we call "Type 2" and "Type 3" which were made at the same time as the Koester's, they just weren't as exhausive as the Koester's set, and instead of only producing the New York players the American Caramel Co. continued to print some of the cards that were from earlier printings along with the new New York players they were including in their product, that is why some other teams (non-New York) and players are found with the Type 2 and 3 backs. Anyways, below find a breakdown of the 52 cards known to be in the "Koester's set"...
The following players were "Only Produced" in the print run witht he Koester's cards, thus if you find these cards they are Koester's cards 100%...
Jesse Barnes
Howard Berry
"Ed." Brown
Jesse Burkett
"Bill" Cunningham
Alex Ferguson
"Chicken" Hawks
Fred Hoffmann
John Mitchell
Chas. O'Leary
William Piercy
Chas. Stengel
The following cards are exclusive to the Koester's Bread set, due to small changes ONLY FOUND in the Koester's set, thus if you find these cards they are Koester's cards 100%...
Lou DE Vormer (Upper case "E" in "DE" as part of his last name "DE VORMER)
John McGraw (Standing pose, Position is "Mgr." on front not "Manager")
Ross Young (Position on front is "R.F." not "O.F.")
The following cards were first part of the E121 Series of 80 Type 2 and 3, Gassler's Bread, as well as the Koseters set--but likely if you are finding them with blank back they are almost certainly Koester's as they were only produced in this fashion during the small print run of the Koester's/Type 2-3 Series of 80/Gassler's Bread print run...
Frank Baker (made in this run and dropped before E121 Series of 120 & later sets)
"Rip" Collins (made in this run and dropped before E121 Series of 120 & later sets)
"Phil" Douglas (P., but they removed the quotes from name in later runs)
William Fewster (Position as "Outf." only made with this print run)
Frank Frisch (Position as "2nd B." only made with this print run)
"Mike" Gonzalez (made in this run and dropped before E121 Series of 120 & later sets)
M. J. McNally (Position as "3rd B." only made with this print run)
Emil Meusel (Position as "L.F." only made with this print run)
R. Meusel (Position as "R.F." only made with this print run)
Elmer Miller (Position as "C.F." only made with this print run)
Roger Peckinpaugh-Correct Last name (made in this run and dropped before E121 Series of 120 & later sets)
Jack Quinn (made in this run and dropped before E121 Series of 120 & later sets)
John Rawlings (Position as "2nd B." only made with this print run)
Robert Roth (made in this run and dropped before E121 Series of 120 & later sets)
George Ruth-Throwing (made in this run and dropped before E121 Series of 120 & later sets)
"Bill" Ryan (made in this run and dropped before E121 Series of 120 & later sets)
"Slim" Sallee (End of throw-made in this run and dropped before E121 Series of 120 & later sets)
"Pat" Shea (P., but they removed the quotes from name in later runs)
Aaron Ward (Position as "2nd B." only made with this print run)
The following cards were first part of the E121 Series of 80 Type 2 and 3, Gassler's, as well as the Koester's set--if you are finding them with blank back they are probably Koester's as they were not produced with any of the early print runs, HOWEVER since all are Pitchers or Catchers their position did not change from the "P." and "C." from this print run into the later print runs as the manufacturer didn't change those designations in later runs like they did with the infield and outfield positions bu shortening all Outfielders to "O.F." and the infield positions from 1st B." for example to "1.B." Also, in theory any later print runs would be on the much thinner card stock.
Cecil Causey (P.)
Alexander Gaston (C.)
Harry Harper (P.)
Waite Hoyt (P.)
Miller Huggins (Mgr.)
Arthur Nehf (P.)
Tom Rogers (P.)
Robert Shawkey (P.)
Earl Smith (C.)
Frank Snyder (C.)
That leaves us with the following cards. These are the cards that originated in the earlier print runs of the E121/W575-1 sets and were then continued to be made with the Koester's set, these are the ones that may or may not be Koester's if you found them. *An interesting aspect is that the W575-1 cards with "Henry A Johnson" stamped backs were only made at the ame time as the earlier e121 Type 1 backs, thus in theory most of these should be found with the Henry Johnson back, which is mutually exclusive to the later print run W575-1/D383 Koester's bread cards above (the ones listed above should never befound with legit Henry Johnson backs)*
Dave Bancroft (Leaping)
Geo. J. Burns
Hugh Jennings
George Kelly
Carl Mays
Walter Pipp
Walter Schang
Fred Toney (Trees in background)
Hopefully this clarifies things a bit, but it may just produce more questions.
-Rhett