Idk of any Williams Brass M1a? but there is someone on da bay claiming to sell a Williams M1a. The pics even show what appears to be a genuine Williams Brass box with a PRR Griff tellereque painting on the cover. The box is the last brass black two piece box that came with the T1 and the USRA Mike's.
None of my saved Williams literature has any announcements for an M1a. And the M1a looks awfully familiar to the Weaver Models M1a that came out in the late 1980's white tires and bright red paint too.