Thank you guys , I have no idea what it is , I took a punt on it really , as it looked cool & I have not seen many Under Lever single shots !

One chap used a tape on the muzzle & rim cut , muzzle was 11mm , rim was 16mm , the other guy said 500 muzzle , so I was hoping for a .500BPE , but if 52-53 is .45Cal could be 500/450 of some type ?

The butt stock has a few chips , cracks & repairs at the butt plate end .