I do not believe these are cut from an ad. There is a minimum of 60 different without the name on the front. The idea that Sporting Life would put none identified players on a sheet of potentially 60 just seems odd and unlikely.
Also the cards have variations - for example some have no name on front and some do. Both blue background and pastel are available in both blank back formats.
The idea of a card being a proof is that the card in question was used for production of the produced card. If these are early production cards wouldnt that in fact make them a proof?
The clearest example for me that these are production cards is the Harry Steinfeldt I picked up years ago. the blank backed card clearly shows the edit of when Sporting Life cleared the word "Cincinatti" from his uniform (as he was with the Cubs in 1910 at time of production). Below is the blank backed card compared to the regular card. The edit clearly shows they needed to update the card for release. This makes the idea that they were on a poster seem very unlikely as well.
My money is on them being early production cards. Whether they can be considered "proofs" or not is rather pointless to me as they are one of a kind early production cards
m116 steinfeldt proof.jpg
m116 steinfeldt card.jpg