peter72,
ONCE MORE, you NEED a chamber cast to see what EXACTLY that the caliber actually IS. = MANY older/antique firearms were rebored/rechambered in the pre-WWI & post-WWI era.
Btw, because of Jessie at JES, reboring/rechambering to a different caliber is NOT uncommon NOW. = I have & would NOT part with a Model 760 that was once a .30-06 & is now a 9.3x62mm Mauser for dangerous game.
yours, tex