Built post-WW1 by the Zella-Mehlis guntrade on an ex-military Kar98AZ small ring carbine action and a civilian, most likely Z-M made barrel, stock and sights. Proofed at the Z-M proofhouse in 1.20 = January 1920, gun number 710 of that month, for a St.m.G = steel jacketed bullet. 7.8 mm was the bore/land diameter (not groove/bullet!), correct for a 8x57I, .318" bullet. CROWN-crown/N proofmarks indicate the special proof using the 4000 atm proof cartridge, according to the rule of July 23, 1893. Most likely a 8x57I, as the 8x60I did not yet exist in January 1920, but make a chamber cast and slug the bore to be sure. If a S, .323" bullet slips free in an out a fired case, you may also use 8x57IS loads. At least, in any case you may use the weak, 170gr at 2400 fps, American SAAMI specification "8mm Mauser" loads as offered by Remington, Winchester or Prvi Partizan. These "8mmMauser" cartridges are loaded to very low pressures, 35000 psi max instead of the European CIP 52000 psi, to avoid any danger if they are used in an 8x57I barrel. Remington even loads them with compromise .321" bullets.