Found a WM in the War Department Technical Bulletin: TB 5-9720-11
Technical data for tractors and other attachments 10th July 1944.
The TB mentions that it was used for towing and general construction.
I would imagine that if you did find a Allis WM that was definitely used by the US army during WW2 that it would have US stamped on the tractor serial number. As seen on ex army Caterpillars, and Cletracs.
(It is quite possible that the OD Allis M is original as the hood number has the correct prefix (9) which is correct for a tractor, although it is possible that the owner just painted it on there knowing this fact.)