It is an interesting card but not likely a promo card given the size. The only promo cards that I know of are a singular Holmes to Homes Walter Johnson, which I call a promo because it is unnumbered even though there is no evidence of a promotion, and the 6 Successful Farming cards mentioned in the magazine’s May 1916 issue. Here is the Lajoie from that group:
There is no evidence thus far that Mendelsohn used salesmen for m101-4/5, unlike the m101-6 premiums where he recruited heavily for hawkers, and no evidence that samples were sent by mail either. I have not held in hand the flyer that was discussed in the old thread from 2012 regarding the OP's Lajoie, but assume as mentioned there that any player shown on it would have printing on the back if cut out even if it was the correct size and stock for m101.
The change in pose is probably best explained by the change in team uniform Nap is wearing, although it might also have been made to show more “action”. Mendelsohn’s ads for these cards focused on players being in “action”, and all or nearly all of the non-manager cards depict the player holding some equipment–bat, ball or glove, or a few in follow-through. Maybe Felix thought a hands on knees pic was too passive.
It is unclear where the OP’s card belongs on the Mendelsohn evolutionary chart, but it should be there somewhere IMO. I believe he was behind the production of what now has been catalogued as the 1915 W-Unc set, but I surely can’t prove it. I also believe he toyed with the oversized version in 1915 of which three are known (mentioned in the old thread), and then settled on the size and style for what we now call m101-4/5. Here is one of the biggies:
Little or no info has surfaced since that thread in 2012 discussing the OP's card, so for now it looks like I would agree that this is part of 1915 W-Unc. That being said, it could be an unissued variation, or the next step pre-production to m101-4/5. Just to stoke the fire, look at this ad for m101/5 that appeared in the Chicago Examiner, showing Matty as part of the set one could obtain:
Such card was never part of either m101-4/5, yet it appears it was ready for inclusion. Note that the same pose is used in the 1915 W-Unc set, although with only one text line,
and that it is also used in the mystery three-card group from that year, with two lines.
So it appears that other cards were either produced or ready to go but were replaced and/or never made it. The mystery remains, and the OP's card just adds to it.