If you have to/want to use it for upland hunting, you can buy PolyWad "Spreader" or "Doublewide" shells that will shoot about an improved cylinder pattern from your full choke barrel. It is worth getting a box for the occasional rabbit or grouse or quail hunt or "target of opportunity" while hunting for other stuff. Cheaper than a rechoke job and keeps that beautiful gun original.
I believe that PolyWad only sells 4-box lots of shells, so you will need to find a friend who has a full choke gun and have him get a box for himself and you can each get a box of slugs. Or maybe get a couple of boxes of their low-pressure, low recoil 16s for practice. A seach for "polywad home page" will get their site, I think.
there is another company that sells spreader loads, but I forget their name.
If you handload for the 16 or know somebody that does, you can just buy the plastic spreader wads from PolyWad, and have them loaded into an appropriate shotload. Check your gun to make sure it has a 2 3/4" chamber; many from that era have Continental 2 1/2" chambers (PolyWad sells those, too. No, I DON'T have stock in the company!). If the chamber of the shotgun barrel is marked 16/65 on the bottom of the barrel, it is a 2 1/2" chamber; 16/70 is the "export" or 2 3/4" chamber we use. I have seen both on German shotguns from the mid-30s.
Neat gun; enjoy!
Mike Armstrong