I have an unmarked drilling with an 8mm rifle barrel that has this same type of marking for the powder & bullet, IE; 2grG.B.P / St.m.G. Its caliber marking however is 172.28/59. Other than the 59 all other dimensions check out for the 8x58R Sauer. It bears a proof date code of 2.07 / 690. Unfortunately from 10" to 13" from the breech the entire right hand wall of the right barrel is missing, from top rib to side rib.
A Hammer Drilling by Miller & Val Greiss has a 9.3x72R rifle barrel under. It is marked K.m.G / 12.7gr for bullet only. Caliber marking is 8.8m/m / 72. Proof date is 6.13 / 240.
Both have regular 65mm, 16 gauge chambers. Both are Nitro Proofed for both rifle & shot barrels. In both cases the "m" for Mantle in the bullet designation appears to me to be lower case, as it does on this example.
