I think you will find that the cartridge you are looking for is the 6.5x53 Mannlicher. The Brits called this cartridge the 256 and in their writing, it is often confused with the 6.5x54 Mannlicher Schoenauer. Both these calibers were popular with the Brits. at the turn of the century.Custom loaders use .303 British cases to make ammo (30-40 can also be used).
Mike