SbySW is correct, but let me point out that it should be an R in an oval, not a circle. It is on all the Colt made 1928 stuff....that these are two piece fasteners, with a inner pin that expands the outer (and thicker) ring? Could I just hammer that inner pin out?
Oh yeah, the unit looks to have been originally blued, neglected and rusted thoroughly, and then heavely painted over the rust. Oh, and beat and banged up throughout.
You've seen abunch of these, do you know what's the circle R stamp designate?