An amazing find indeed, Argo44. The first Joseph Lang breech-loader might have been produced in 1853 or early 1854; John Blanch stated that he built his first pinfire in 1856, and from your research Reilly began in 1856 as well (unless future research pushes it back even earlier).

What a delight to see such an early British pinfire, cased no less. The long underlever is very interesting. A rare, rare gun.