Bill,
I would e-mail or PM Rhys (same username for Net54) and I'm sure he would be glad to explain his reasoning. The posted scan is too small for me to make out whether he has an 'M' on his cap (though the image may be too washed out in that area to tell anyway). The back of the photo does have 1921 and Baltimore pencilled on it, but every image I can find of Baltimore players from either 1920 or 1921 has the black portion of the sock extending all the way down. I don't have an image of any Martinsburg players for either of those years, so I don't know that that proves anything definitively, but there may be some other supporting evidence for Martinsburg that I can't see or am overlooking. Either way, Rhys is a good guy, and I'm sure can shed more light on it than I can second-guessing him.
And if you would, please post your findings back here, because now you've got my curiosity up
