I have encountered just two such Haenel Schonzeitbüchsen until now. One of them was rechambered to 6.5x57 by some idiot post-WW2, the other one had the barrel set back and rechambered to .250-3000 in the USA. There was another .25 Rem rifle by Otto Bock, but on a Mauser 98 K = short action. Another, indirekt hint to the use of the .25 Rem in Germany: somewhere I have a "drawer find", a W-W box of .25 Rem loads, most American, but 4 or 5 by DWM. The .25-35 Win aka 6.5x52R was a popular varmint/ small game, up to roebuck number here in Germany before WW2. The .25 Rem aka 6.5x52 found the same use, but such specialised bolt action rifles were real odd.
If you can take the rifle home, please post photos of the rifle and scope. Maybe I can tell you something about the make and age of the scope too.