This Greifelt rifle was proofed 12 35 = December 1935, obviously by the Suhl proofhouse. So the rifle is dated. 6.9 mm is the bore/land diameter of the barrel, not the groove diameter. At the proofhouse a cylindrical 6.9mm plug passed the barrel, while a 7.0 mm one did not. So the real bore diameter may have been as large as 6.999 mm. You have to add 2 times the usual groove depth of .1 mm = .2 mm to arrive at the groove/bullet diameter. The “7 mm” cartridges are named so for their bore/land diameter. “7 mm” bullets are actually 7.2 mm = .284”, as you know. The rifle was proofed for a nitro service load using the then popular 9 gramm = 139 gr steel jacketed bullet, essentially the same as the .275 Rigby High Velocity load. 57 (mm) is the case length, of course. The crowned letters stand for: B = proof load fired, U = inspected/viewed for defects, G = proofed for bullet use, N = smokeless proof.
I cannot make out the “crowned oval” stamp. It looks similar to the Liege, Belgium proofmark, but is most likely the mark of a Suhl barrelmaker. (import of firearms and parts was prohibited during the interwar years, and the Liege proofhouse applied this mark to complete firearms only) The other numbers and letters under the receiver, 12 and ND/5MYG/b are leftover factory internal control stamps from the original making of the Kar 98AZ carbine receiver at one of the German government arsenals, most likely the Danzig factory. They are unintelligible now.