Drilling Caliber ID

The rifle barrel is stamped 6/3 mm, it is a rimmed cartridge. Anyone familiar with this caliber designation? 6.5x53R ? Probably have to do a chamber cast but thought I would ask first. Any help will be appreciated. Jim

Already answered GB,
6.3 is Belgian indication for 6.35mm ie, .25 cal. As in 25/35 WCF.
Cast Chamber to confirm.

Doc AV

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Or a 6.5x53R Dutch Manlicher, maybe ?