Raimay, ".366 Mauser" was simply the British name for the 9.3x62, just as they called the 7x57 ".275 Rigby" or the 9.5x57 M-Sch ".375 rimless nitro express"!
Yes, the proofhouses changed over from the gauge system, often employing decimals like in "108.49" to the metric system for rifle barrels, Zella-Mehlis in 1911, Suhl 1912. At that time Loewe was already DWM, who got their gages and measuring instruments from the gage shop of their subsidiary company Mauser. The proofhouses did not have to go as far, as both Suhl and Z-M were not only gunmaking centers then, but had also as many companies making precision measuring and surgical instruments. Alas, any gunsmith past his first apprentice year should be able to turn those simple cylindrical steel plugs.The GDR closed down gunmaking in Zella-Mehlis, but several gage-makers to the industry still exist there.