After all the speculation here, now I'm wondering if that's a welded back together receiver. The pits described by ct241 are right about where they would be. We'd need to see the other side clearly, and maybe the inside for more signs. The scrubbed heel would sort of figure, and the number looks to have been engraved. All the numbers are perfect, look at the 5's... The whole thing has been re-blued(or parked), bayonet lug polished off, front sight blade slimmed and who knows what? Even the barrel marked look to have been reduced...