That's the thing... There really are no other markings. I had jreifsch80 look at it, and Stan May look at it. They both said it's Spanish and small ring for sure. I have just read that the early ones are better than ones made after 1940? I really don't know anything about Mausers.

It does look like the other caliber suitable would be 6.5X55. It does sound like an interesting caliber. Just wonder about the availability.