These are not factory markings, but the usual set of German proof marks, not directly related to the cartridge designation. Until the 1920s German and Austrian proofs were not respected vice versa. So Austrian guns had to be proofed by a German proofhouse according to German proof rules before sale in Germany. From 1912 up to 1939 the German proofmarks give these Informations: Bore/land diameter (NOT groove or bullet) of the barrel, measured with cylindrical plugs in .1 mm increments. As the minimum bore of 6.5x54 MS barrel is 6.48 mm, a 6.5 mm plug did not pass the barrel, so the 6.4 mm marking is correct . The groove/bullet diameter is 6.7 mm = .264". 53.5 mm is the real length of the “x54” case. The rifle was proofed to use the standard 6.5x54 M-S service load, a 10 gram St.m.G = 154 gr steel jacket bullet. So there is nothing mysterious here, just the contemporary German proofmarks for a 6.5x54 Mannlicher – Schoenauer rifle.