The marking of the German proof house indicated the "Bore" dia, rifling not included & length of chamber. It was not marked as to whether the cartridge was rimmed or not. As to the caliber 6.5mm = .256" which is approximate bore dia foe the .264" bullets. It is actually very close to actual bullet dia for the .25's IE .257". I understand the .25-35Win round became quite popular in Germany for use in drillings & combo guns, they apparently took the liberty of calling it the 6.5x52R as a play on the popularity of 6.5mm rounds throught Europe, there were few actual .25 caliber rounds produced there.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra