While waiting for photos, from your description the rifle was apparently converted, rebored and reproofed, pre-1940 in Germany to 9x57. 12.7 gramm was the bullet weight of the original 8x56 M-S chambering. 8.7mm is the bore/land diameter (not groove or bullet!) as determined by the proofhouse by slipping cylindrical plug gages into the barrel. 57 mm is the case length. 18 gramm was the heaviest bullet weight of the 9x57 cartridge. Austrian proofhouses did not stamp such information. There should be also crown/B, crown/N, crown/G and crown/R proofmarks on or under the barrel.