It's an 8mm. Based on the vintage (22,000 SN range) I assume that meant 8x57 when it was manufactured, and an 8x57 will chamber. Unknown if it was later rechambered to 8x60. Apparently the ring is round...don't know how it is marked but the left wall is marked Waffenfabrik Mauser-Oberndorf and it has the commercial BU proofs.

I asked for a photo of the peep sight. Verbal description is that the peep and bridge have a filed cross-hatch pattern. Sounds like quality work but with a very limited knowledge of commercial Mausers and one so-so picture to work from it's hard to make any judgements on originality.