No gunsmith here, but am curious. Is the ammo still good ? Or, what does the primer strike look like after you pull the trigger? That is if the round is a typical centerfire. I've seen week struck rounds before, that did not fire, last one was an M1 carbine that I can recall. You could see the difference.
The other thing is not being able to confirm the caliber unless you could compare a detailed sketch of the round and the chamber ? How do they do that in a situation like that, if there is no real marking, or any information to go by?
Interesting dilemma on an old mauser, kinda why I am curious, always enjoyed working on firearms since a young kid, cleaning, maintaining, as well as minor repairs, nothing risky, if I won the lottery as many say, I think I'd go to gunsmith school, be nice even if no lottery LOL, very interesting to me!