Below are the photos of the proofmarks, although the caliber mark is 7.9 the bore slugs 3.23 so it is an S bore gun. I wonder if this is a pre WW1 rifle?
The rifle was made prior to 1912, considering the configuration of the proofmarks. Rifles marked with only an indication of powder charge and bullet type as shown below date to 1912 or before:
2.67 g GBP St m G
Now here's where it gets interesting - note that yours is marked 2.75, whereas most 8mm proofs (that I've seen) 2,67 grams of Gewehr Blättchen Pulver (military flake powder) and Stahl-mantel Geschoss (steel jacketed bullet).
Other possible types are "K m G" for Kupfer-mantel Geschoss (copper jacket) and "Bl G" for Blei Geschoss (plain lead bullet).
