I expect this one is made in the US - thus not a Howdah pistol.
The styling of this pistol reminds me very much of Henry Deringer's work in belt or holster pistols. I am surprised it is not marked with the maker's name - thus again, not a Howdah. Howdah pistols were of higher grade and of English make as far as I know - for carry while hunting in India.
Howdah pistols were of large bore - .54 to .58 is NOT a large bore, but a normal size for a coach pistol or holster (horse) pistol in the US. The only .577 calibre ie: small bore Howdah pistol I am aware of (not to saying this is definitive proof) is a .577 Snider 4 barrel pistol of the ctg. era- ie: .577 Snider ctg.
.54 calibre pistols, mostly military but civilian as well, found much favour when shooting buffalo from horseback. The saddle carried two pistol holsters, one on each side at the front, for the holster pistols.
A .54 or .58 round ball would not be very effective "in the Howdah" where they were used for repelling tigers who were trying to kill the hunters in the Howdah.
Muzzleloading pistols made for the Howdah were of large bore, generally 16 to 10 bore - with 2, side by side barrels. Pedersoli makes a modern Howday, of 20 bore.
I agree that the breech/fence/hammer relationship looks to be English as it is correctly designed, however the US makers also knew how to breech cap lock firearms properly back in the 1800's.
It was only with the reproductions in the 1960's onwards, that proper breeching knowledge went missing in the interest of cutting costs of mfgr.
In my opinion, of course.