Sir what a beautiful Rook and Rabbit rifle you have! On looking at the pictures I thought oh it’s a Sherwood .300 caliber but having stopped being lost in the blueing and quality I read your accompanying text.! Was unaware of such a cartridge. NEVER TOO OLD TO LEARN,!
I looked in Bill Flemings book British Sporting Cartridges, Barnes Cartridges of the World still none the wiser.
Could the rifle have been relined at some point in the distant past to a rare experimental cartridge.
This is definitely one that requires homework good luck with your endeavours and keep us posted please. Regards Steve.