Before WW1 the Austrian proofhouses marked the smaller bore/land diameter on the barrels, as did the German proofhouses until 1939. In the 1920s the Austrians changed their marking habit to the groove/bullet diameter.
Most of the European “6.5 mm” cartridges were named for their bore diameter, F.i. the 6.5x54 M-Sch, 6.5x55 SE, 6.5x57(R), 6.5x68. The British called them “.256” too. But they all have 6.7 mm groove barrels and use .264” = 6.7 mm bullets. Exceptions: The 6.5x52R aka .25-35 Winchester uses 6.5 mm = .257” bullets, while some obsolete cartridges like the 6.5x54 K Mauser or the 6.5x58R Sauer & Sohn used 6.64 mm = .261” bullets. You may sometimes find old Czech 6.5x54 M-Sch ammo headstamped “6,7x54”for their true bullet diameter.
That grip cap well holding a spare frontsight was a pre-WW2 Steyr factory option of their “English model”, the take down rifles. The parts, K1 – K10, after a 1929 factory flier parts list: