Now before you go ballistic and say GM, it's a shotgun.....look real close, and remember, it is still completely German.
This is a highly modified and converted Mauser Gewehr 98 rifle, and judging by some of the faint markings, I'd say possibly an Amberg. These were modified in Germany in the 20's through the 30's, along with the name Geha, there were two other makers, Remo and Hard Hit Heart. They can be found in 12, 16 and 20 gauges. Even the stock as you will see is a military stock, it still has stamps present. Now, given the age, the 12 is only chambered in 2 9/16 or 2 1/2, using an American 2 3/4 shell could and will not end well, in other words, it could kill you. The 12 you will notice, it uses the safety lug of the 3 lug Mauser action to lock the action, the front 2 main lugs are machined away due to the bore diameter. Using the safety lug in of it's self is fairly safe given a shot shells low pressure, BUT, using the modern longer shell in the shorter chamber could easily exceed the pressures. The mag is fully functional as well, it will hold a shell or two and reload as you cycle the action. The butt pad from what I understand has a plausible need, don't know how old it is, this thing only weighs like 5 or 6 pounds if your lucky, and reported recoil is rough.
Also the bolt head which is commonly missing to adapt the 8x57mm bolt to 12 gauge is also missing, see the last photo, it's an adapter that kind of fits up in the bolt. usually lost when cycling the action really fast. I've got a replacement coming in the mail. Firing with out it is also dangerous, as the pressures could blow the end of the shell out, again not ending well. While cleaning it, I did find the remnants of a shotgun primer in the bolt, which means.....
So all and all.....it's a wonderful old German rifle that could kill from both ends
Think I'll just fix it and put it with my Swiss .41 rimfire.. no need to go for the Darwin Valedictorian award