Off the cuff & without an image, I'd say it is 6,5x52R, being almost identical to the .25/35 but typically loaded to a bit less pressure than the US round. The numbers 1037 over 331 could be a Zella Sankt Blasii - Mehlis proof date & ledger number being Nr. 331 for Oktober 1937??

Serbus,

Raimey
rse