No typo!The 8x51 (Mauser K) cartridge was a special cartridge for use in the short Mauser K = kurz = short actions like this rifle, introduced by Mauser about 1901. A .30-30 class cartridge, it was intended for use on roebuck and other small deer or antelope. It was chambered in the K-action commercial Mausers and the Schmidt & Habermann M21. It uses .318” bullets. In the 1930s it was offered by DWM only. Two loads were available then:
157 gr jacketed rn, 38.5 gr R5 smokeless for 2156 fps
196 gr j. rn, 38.5 gr R5 for 2100 fps (20” barrels)
Obsolete since WW2, it is a pure handloader’s number now. Cases may be formed from .308 Winchester brass.