Coincidentally, I had been working on my much modified #3083178 when I read this. The floor plate and the guard are stamped "R" in the same place as yours. The stamp on the guard is poorly done in that the loop of the "R" and the "hind leg" didn't make it, but the stamp is there. This leads me to wonder if the earlier guards were so stamped.

Incidentally, the hole in the tang for the rear guard screw is drilled through on this rifle; I was not aware that modification occurred so early.

I also have rifles with numbers starting with #3026, #3045, #3050, & #3012. When I get to it I'll check for guard stamps on those.