Try googling "India Pattern Brown Bess". The regular brown bess had a slightly heavier butt plate. The very first of the India Pattern was not built to mil specs, it was built originally for the East India Company in 1797 but the English government needed a bunch of guns ASAP and took them (bought) from the East India Company.
They went on to use them up to 1854 when they where disposed of all over the world. Mexico used them as late as 1830s in Texas against the US, and lost! Bringing a flintlock to a repeating gun fight, go figure!
Many where disposed of in Newfoundland and my friends have found several of that shape and design. I am the relic guy for my group and I have researched them. They where sold and used for hunting sea birds and where we found them was a famous spot for hunting eider ducks.
A famous quote concerning the accuracy of the India Pattern was from an officer who said that a man standing 100yds away would have little to fear from a shot from one. They where fairly lethal at 25 to 50 yards but had no rear sight. The front sight was need to attach the bayonet.
HH
1859
Come to think of it, we don't find any gun brass with these guns. Taz's gunis probably be the correct fit for this butt plate. I don't have them nearby and I don't have a picture with me. I don't recall that tang on the 2 we found. So your gun is probably earlier than the "India".