Also found this.
There is a mark on the lock-plate - a crown with letters DN underneath, which possibly stands for Delft Nederlandse. Seaside city of Delft was a location of one of the largest Dutch arsenals. Another possibility for the DN abbreviation is relating to the O.P. Drissen & Cie, a known Liege gun maker.
Barrel is 23 cm long, in man-stopping .69 caliber
Also this.
DUTCH Antique Sea Service FLINTLOCK Pistol
NAPOLEONIC WARS-Era Military Made in Liege
Here we present an antique Dutch Sea Service Flintlock Pistol, made circa 1800 in Liege. Today we know that Liege is a city in Belgium, perhaps most famous for its centuries of arms manufacture; but at this time, Belgium was not yet independent, and was a parcel of the United Kingdom of the Netherlands until recognized in 1839. This period in history was dominated by Napoleon Bonaparte and his wars of conquest. Interestingly, the Netherlands was a part of the French Empire from 1795 to 1813, and Napoleon renamed it the Batavian Republic, then the Kingdom of Holland, according to his purposes. During this time, the Dutch military fought alongside France against Napoleon’s many enemies. After Napoleon’s defeat in 1813, William I was restored to the head of the government, and in 1815 he declared himself king in the face of Napoleon’s return from Elba. William I’s son, the future King William II fought Napoleon at Waterloo.