This is the engraved silver thumbpiece on a 16ga. double I recently purchased. It is converted to percussion from flintlock. 32 1/2" barrels, 15" pull to front trigger. It was made by Barber & Boaler, who were a partnership 1800-1817; proofmarks are pre-1813.
The mark is a crown over a
caboshed stag's head, over a coronet. Research suggests that the crown over stag's head is the crest of the Legge family of Dartmouth, the coronet is the mark of an earl.
William Legge, Fourth Earl of Dartmouth, was born in 1784, died 1853. He was a young man when this gun was made.
Antique guns are fascinating; being able to associate a person with the gun makes it even more interesting.