The underhammer rifles were developed in New England in the 1830's and some folks swear by them for accuracy and durability, The Ethan Allen / Allen Thurber rifles were first made in the late 1830's so both guns will somewhat fit the pre-1840 theme though some particular models may have been made later, a little research may be in order here if you want to dial things in super fine.I had an Allen Thurber and it was an excellant offhand gun, the lock mechanism is simplicity to the extreme, both guns would probably be good choices for the late (pre-1840) period.