Let's hope Purdey's will sort it out. My suspicion is that the barrels could easily be original. It has London Black Powder proof marks of the correct period and later Birmingham Nitro Re-proof marks. I have an 1869 Purdey with the same swamped rib that is stepped down from the action. I think the best clue for Purdey's is the CO with the crown above at the breech end of the barrels. If this matches one of their workmen's marks; probably the barrel filer. I don't think they were having hammer guns made in Birmingham of that quality at the time. As for Island locks or normal back action locks that would be no indicator other than customer preference. Lagopus.....