If it is what I'm thinking, its called "close plating" or "Sheffield Plate". It was a thin layer of silver soldered to a base metal. Copper was commonly used but there is no reason why brass wouldn't work. It is possible a gunmaker might have done it but the process was a closely guarded secret. Its possible that it was made from plated "sheet metal" which was the raw material turned out by the Sheffield mills.
For a long time the only Sheffield plate made in America was done by a Connecticut clock maker whose name escapes me... his apprentice, Seril Dodge (that spelling is actually correct!), carried on the practice in Providence, RI, making his fortune with Sheffield plated shoe buckles. Dodge is also believed to have made a number of hangers with Sheffield plated guards. There are at least two or three of these extant with a solid Revolutionary War provenance so the process was known at least in the 1770s.