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 Statesicon Marine 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.