Curious though, with the S-A 11-46 on the barrel, I would assume that the M1 had gone through a Gov't (possibly Springfield) Arsenal refit and that the barrel was of Springfield manufacturer. The gun shop selling it didn't think the USMC stood for United StatesMarine Corps, but would there be a possibility that Springfield may have started to replace barrels for the Corps on a limited basis, marking them, but then suspended the program. If so, I would think there would be some mention of it in Duff, Collector Books, and elsewhere. I thought maybe it stood for Untied States Marlin Company or Corporation, but from what I understand Marlin marked their barrels with just 'Marlin'. Beyond my 'pay grade' of M1 knowledge.