St. m G
Means proof tested with 2.67 grams (41.2 grains) of flake smokeless powder and a steel jacketed bullet. Standard mark for the 7.9x57 1888 which agrees with the .318" bullet. (Except there should only be on letter B in the powder designation, but the proofhouse clerk was probably overworked and hit the stamp twice.)
Krupp was the big steel company in Essen, Germany.
Kahles was and is still a good brand of scope sight, now a division of Swarofski.
"Not a Mauser?"
Show pictures. It might be a sporterized 1888 which is not really a Mauser. it might be a Mannlicher in 8x57 instead of 8x56. Who knows? Time for Show and Tell.