This pistol was in use during the reign of King George III, who reigned between 1760-1820. He is most known for being England's King during both the American Revolution and the Napoleonic Wars.
The lockplate is marked TOWER for the Tower of London as well as a crown with GR for George III, as well as the Broad Arrow mark with a crown over it, signifying it as property of the British crown.
The left side of the stock is stamped with a maker's mark, "TB" for Thomas Barnett, a gunmaker located at 134 Minories, London from 1796-1810. Barnett was a contractor for the Board Of Ordnance from 1793.
The gun bears the crossed sceptres and crown proof marks on the barrel, also called the King's Proof, in use in England after 1710 for military firearms. These consist of the letters GB over a broad arrow and a separate mark with two sceptres crossed with a crown over them. There is also a reversed P over these marks, the meaning of which is unknown. Also on the barrel is the number 19 with a crown over it, likely an inventory number. The stock is also stamped with the letters GR in script letters with a crown over them and the numbers 180 - likely 1800 with a V underneath. An example of a very similar gun, also by Barnett, in the collections at Fort Ticonderoga, New York lists this mark as a storekeeper's mark. The ramrod is also stamped with a crown.
Overall a very nice example in good condition with clear marks for its age. The lock plate is approximately 5 inches in length and the bore is .66 inches.
