It's a custom gun.
Built by CG Haenel They have been building guns since 1840.
Suhl Germany is the city where CG Haenel is located
The rifle is based on a Mauser 1888 Commission rifle, with LOTS
of custom additions and refinements.
Good luck finding the scope mount that fits it---they were pretty
much custom built to the gun.
Caliber---no guess. Have a chamber cast done and start measuring.
I'll bet that under that copper paint there is a REALLY nice piece