Thanks for posting, Parabola.
According to the IGC Database, the Leech family gunmaking business in Chelmsford possibly dates as far back as the 1770s. In 1839 the business name was changed to James H Leech; in 1858 James’s son, William, was a journeyman gunmaker under his father, and after 1862 he became a partner in the business and it was re-named Leech & Son. The Jones-type double-bite mechanism was widely copied after 1864. So, your gun fits in nicely with the pinfire timeline, and I don’t see signs of it being a converted muzzle-loader. George Thomas Thorpe Bartram came later, and while in the gun business he was also a china merchant. He may not have made guns himself, but rather bought guns from ‘the trade.’ It could be that Bartram bought the gun from Leech & Son for re-sale, though re-barrelling is also a possibility. Either way, you have an interesting gun in quite decent condition, with names that have not so far appeared in this ever-lengthening thread.
Always a pleasure to see surviving pinfires!
