given the .510 rim diameter...and the 6.3 marking, i would suspect it is 25-35 winchester. they saw quite a bit of use in that era, and prior to the great war, a lot of germanic guns were marked with barrel diameters of the lands, rather than the "true" bores.

i had examined an older drilling not long ago that was marked 6.3 x 52r, and slugged properly with .257 bullet.

best regards,

tom

"it's a poor sort of memory that only works backwards."
lewis carroll, Alice in Wonderland