Don,
The "0" with the crown is probably actually a U.
I would suspect that it is an 8X57 which is quite a common chambering in the prewar Oberndorf Mauser sporters.

There are many good books on these guns and it is quite easy to date these guns.
Go ahead and post pictures. These Oberndorf Mausers are all very collectable and desirable especially in original configuration and condition. I am sure many of the Mauser experts will chime in.