Wilhelm Kelber was a barrel maker
its a Suhl made rifle from the 1920s to early 1930s

Max. CIP bullet diameter for the 9x57 is .3567. there are some 180 -200 grains .357 bullets for the .357 Mag I can recommend you for start to reload the 9x57 Mauser. 8x57 IS brass is fine and the CH4D die set is everything you need.