I'll guess 8x57 mm JRS, which is simply a rimmed version of the 8mm Mauser (8x57 mm JS). The rimmed version was used in combination guns and single shots.
The round was introduced in 1888 with a .318" diameter bullet, but was adapted to .323" in 1905 to correspond with the change in the military round. The "S" or "JS" suffix is used to denote the larger bore diameter. The "R" is for "rimmed" of course.
