Hi Muddy,
Branders and Potts worked near the Minories in London between 1802-1827. They were contractors to the British Ordnance department. I agree with you that it may be a sea service pistol. There was an HMS Duke of York during the 1760s but also 2 commercially built armed ships of that name hired by the Royal Navy at the end of the 18th century and early in the 19th century.
dave