It's an English style target/holster/dueling pistol. The half-stock style appeared circa 1800-1810 (maybe as early as 1790) and was very popular, both in England and the US, within a few years, so time wise I'd put it circa 1810-20 or even later, but the percussion system had been generally accepted by 1830 or so, especially in pistols and double barrel smoothbores.
The flint lock on this one is of an earlier period (circa 1780's?), but that style remained in use on less expesive pieces well into the 1840's. Higher priced guns were almost all using the improved locks with waterproof pans, roller frizzens, etc. by the 1800-10 period.
Jed Smith was carrying a pair of similar styled conversion pistols when he was killed along the Santa Fe trail in 1831.