It looks like a “Ketland” style...
Saying a "Ketland" style is same as saying English import lock since the Ketlands made many different style of locks, none of which were any different than other English lock makers were producing for export at the time.
It could date as early as the 1780s. A good date range, is 1790-1830.
We're just guessing on the date range because of the uncertainty of the original shape of the lock tail and because it's missing its ****--two things that help to date locks.
But for edification purposes, the fashion in English lock shapes changed from a long pointed tail such as seen on Champers "Early Ketland" to a tail with rounded shoulders with a point to an even more rounded tail with a "***" or small point like on a Chambers "Late Ketland" to a fully rounded tail.
For import locks, the Chambers "Early Ketland" is representative of the Colonial Period and the Rev War (even post Rev War because the British government didn't allow export of guns and gun parts to the US until the 1790s), while the Chambers "Late Ketland" is more a post-1800 style of lock.
To better illustrate this transition in the shape of the tail and provide a better estimate of time, the US government contracted with the Ketland firm in 1795 to provide 3,000 musket locks and 3,000 rifle/pistol locks. The rifle/pistol locks were delivered over the period from 1797 to 1800.
From George Moller American Military Shoulder Arms, Vol. II, page 10.
Here are some pictures of these Ketland contract locks that were used on pistols built from 1797 to 1800. They are identifiable as being part of the 1795 contract locks because of the "UNTIED STATES" stamp on the tails of the locks.
It's worth noting the subtle differences in each of these locks--no two are exactly alike. But my main point is that the tails of these locks have a longer, more drawn out point on their tails than the post-1800 Chambers "Late Ketland" shown in
Loyalist Dave's post above.