Serial number R3776 (looks like a P but it's a Cyrillic R), Izhevsk factory mark, 1951 year of manufacture. That is the full way to identify your unique gun.
The stamps of squares or diamonds with various configurations of lines are put on during re-furbishment. Each army arsenal whacked their mark on a piece when it went through their work-shop. I see a lot of the square with a vertical line, but the concentric rectangles is not one I know. There must be a page out there with a chart of Soviet arsenal markings?
The 8 way up in the corner is likely an inspector's mark.