I can't remember what Government firearms parts they were, but I have a memory that the Government had parts that were hardened stamped with an"H" . Maybe the frame, or the disconnect stop were originally unhardened but that use found the part needed to be hardened, and later production was stamped with an H until such time that the H became irrelevant.
No idea of the R. Lots of guesses: "right", "radius", or "radishes".