The pistol looks English to me, based on its fine checkering on the grip, single set trigger, and slotted nut on the tumbler shaft. But the barrel looks rough for a British gun and should have proof marks back near the breech.
Jerry Noble's "Gun Trade in America" compendium of all American gunsmiths has a listing for a "T. Atwood" marked rifle, otherwise unidentified, suggesting there was such a gunsmith working somewhere in America. I also checked the "Canadian Gunsmiths" book due to the English influences in the pistol, but no Atwood was listed. There's always the possibility of an English import pistol later marked here in the States by the importer.
Could we get a clearer picture of the barrel name and the lock name?
Shelby Gallien