This rifle was made in 1919 by the former, until November 1918, Royal (Prussian) Rifle Factory Erfurt, renamed Reichswerke Erfurt, RwE. The factory tried to convert to civilian production after the armistice. As Germany was in revolutioary turmoil then, the proofhouse system was out of function. So they used their own established military acceptance personnel for proofing these rifles. In lieu of a civilian proofmark they continued to use the Prussian military eagle mark. Under the conditions of the Versailles "peace treaty", the factory was closed down and dismantled in 1920.
As Lancaster wrote, all those rare "RwE" Mausers were in 8x57 IS originally, though some were rechambered to 8x60S or rebarreled later. Making any 8x57 ammo was prohibited by the Versailles treaty too. As this rifle retains it's original 8 mm barrel, it is either in 8x57 IS or 8x60 S. Forget any other exotic chamberings for cartridges unknown or unavailable in Germany then. When measuring case length from a chamber cast remember: On a M98 Mauser barrel, the rim and extreactor groove of the case protude from the breech end. So you have to add about 3 mm to the distance from the step at the rear of your cast and the case mouth to get the real case length.