There are a number of very odd things about that pistol. In the first place, I am reasonably certain it is not the product of either of the two Ketland firms. It may be one of the Belgian-made copies but those usually - if not always - carry conventional Belgian proof marks. Also, the lock is not marked in the way the Belgians did. The only K guns marked simply "Ketland" are, as far as I can tell, the very earliest made by Thomas Ketland Sr. before he formed his partnership with William Walker. This would be, roughly, 1760 to 1776 and this gun is much too late to fit into that category. The later guns are all marked either "T. Ketland & Company" or "W. Ketland & Company" with a few very late arms marked "Ketland & Walker" and "Ketland & Adams"
The "bun nut" fastening on the hammer appears around the early to mid-1820s. So far, I've seen it on only one K gun, a percussion side by side double that must pre-date 1831 when the W. Ketland firm went out of business. I've never seen a real K gun with a detachable brass pan. This was an exclusively continental European practice (copied by the US) and is virtually unknown in British guns.
The trigger guard is a conventional British design, widely used but the other mounts are strange. If I had to guess, I'd say it was made up quite recently from miscellaneous parts and that the lock is a reconversion.