Letter stamps inside frame

[Strider]

Hi Everyone,
This past weekend I decided to get my 08 out and do a thorough cleaning on it and came across a couple of things that I wanted to share with eveyone. It is a 1940 Mauser 42 code.

The first thing was when I was cleaning the frame I noticed that there are what appears to be two letters on the inside of the frame located at the rear of the frame where it curves back from the handle. The letters are U and B one above the other. I do not know if this has been brought up before and was curious as to what they represent. Are they acceptance code or manufacturing? Has anyone else notice this before and what letters have you guys come across?

The second thing is when I took apart the firing pin. I wanted to clean the inside of the pin to get out any gunk that may have built up. Well I was not sure as how to go about doing this until I noticed a cleaning rod pad for a 22 that I had. What I did was place a couple of gun patches w/clp over the end and used that to clean the inside. It worked quite well. What other methods has everyone used to clean this up?

Thanks.

Sid.