Like Frank says they are authentic W572's that were believed to be "stamped" on back with an ad for Neilson's Chocolate in order to deceive.
I have never personally examined one myself and have also wondered about their authenticity...but I am not privy to the info that Lemke used in the SCD write-up, so I am unsure if it was proven that these were falsely stamped or if they have info as to who was doing it, when they were doing it, or anything else.
Without having all the known details surrounding the W572 baseball players story I would point out that it wasn't without precident that Neilson's would create smaller cards like this... there were two different car sets produced by Neilson's around the same time (one in B/W on front & one in color which share the same style back)...see scan here comparing size of W572 (well cut) to the car series (I forget the ACC # for the cars):
Please note: the supposed "stamped" baseball backs are completely different from these car backs, I bring up the car cards only to compare the sizes of the two sets