The shot barrels show a crown R proof, or "repair" proof. This could mean it was changed from 65 to 70mm chambers, or it could be for some other " repair". There may be a crown R above the 7.6, on the rifle barrel, also. This may or may not indicate the rifle was rechambered. More information, better photo(more)and/or chambercasts will be needed; or better answers can't be given.
Mike