Best idea is to contact the Makers. The record book is fairly complete. http://www.wrichardsguns.co.uk/ Looks like it could have been a pin-fire conversion. I had a 20 bore W.Richards that had been converted from a .450 double rifle. I sold it through Holts a couple of years ago. I was disappointed when it only made 550 as it was in excellent condition and a good looking gun all round. Lagopus.....