Fresh new Luger owner checking in

[Eugen]

I thought I'd show a pic of the backside of the grips. The safety side has an "R" stamp and the other side has an "S" stamp. What do those letters mean? Are these original or replacement? Sorry about the upside down pic, I can't seem to get it oriented properly after repeated attempts.

So much to learn about my new to me 1939 Luger. I am having fun.