For what it's worth, the "390" is some manner of museum mark, not original to the pistol. 
The pan is faceted, not round 
Barrel measurements show that it is not a cut down musket. The profile is identical to a light dragoon pistol, not to a Brown Bess. 
Otherwise, thank you, Gentlemen, for looking it over.
		 
		
		
		
		
		
		
		
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