80 meter was a common distance to sight rifle's with the iron sight's

dwm made two loads in 8x57R 360:
12,7 gramm roundnose softpoint at 662 m/sec
10,2gramm roundnose softpoint at 703 m/sec
with a 68 mm long barrel

can you show the bolt head? if it was a sporterized Mauser M 71 the 8x57 IR A-base would be ideal because its excact the rim of the 11,15x60R. but they made a lot of scaled -down M 71 actions in suhl for hunting and target rifles in 9,3x72R, 8,15x46R and 8x57R 360. you may know that all 3 rounds are based on the same 360 BPE 2 1/4" mother case.

if you can bring the barreled action( the rest of the riofle could be stay home) to berlin I can help you with the claw mount. the gunmaker who had made the claw mount on my mosin sporter( you maybe hear of this) is 30 km south of berlin. if its discussed with him before it will be done after 1-2 weeks and you can take it home again.
you only need an period scope, he have old claw mount parts or make it if necessary.

cost +/- 200 euro but if you prefer soldered half rings it will easy double the price I fear