I saw that same US marking on a M12 that was returned from South Americaicon on some importation (it was in horrible condition). It also had a big sunburst thing next to the US. I have no idea what that was all about.

I don't think the US on there is stamped. It is done with an engraving type of machine, called a pantograph or something like that.

You can not stamp hardened steel - at least not properly, as the fakers usually find out. So that gun was possibly arsenal or depot marked after military acceptance.