Considering how one would process the tooling operations for the receiver, I can see the markings would have required three stations along the assembly process.
1. Markings on the receiver flat
2. US Property mark on the receiver angled flat. This could not have been done at the same time as the receiver flat due to the angled receiver surface.
3. Serial number on socket
Given that the markings were a three part operation, i can see where the US marking might have been overlooked and later ignored if discovered. Like any volume series production line--manure happens. You don't want to know how sausage or automobiles are made.