there's really nothing odd about the 1/2 stamp as similar to the 8.15X46 1/2 R there are a few variations or notations:

6.5X58R(S&S) Sauer
6.5X58.5R

both which are based on the 360 BPE.

Can you post or email a pic of the marks?

Kind Regards,

Raimey
rse