the most used brass in this days was the 11,2x60R Mauser case, we know over 100 hundred different wildcats and a 11x38R is under this. Till you bring measurments of the cartridge I will believe that this is from the Mauser cartridge family. very rare chambering and cartridges, most times only made as a wildcat from local gunmakers. the big firms like RWS making every brass you need before ww 1 and the ammo was loaded from the gunmaker for the rifle.