Yes, thanks again for all the help! I'll see if sometime I can get the conservator to let me take the barrel off and just see what, if anything, is under the stock. I have only handled one presumably Kalthoff weapon, a magazine breechloader type, but I don't remember any barrel markings at all. It was some twenty years ago tho...
Here is a link to a fowler that has a guard much like what is (was, before modification) on our Kalthoff barreled piece. This is just what I thought our gun might have looked like - till I found out the barrel is a bit earlier. Maybe its been on three guns, not two...
http://www.aaawt.com/html/firearms/f398.html