FWIW, every reference I have at my disposal indicates that the Colts were issued with their frames plated with some other material, beit gold/silver/nickle - so, you might want to check further with a Colt historian.
AFAIK, The plated Sharps Pepperbox's were made with brass frames, but usually nickle-plated, or of CCH/steel - so a non-plated brassie may be authentic.
http://www.cslib.org/colt.htm
http://www.coltsmfg.com/archive.aspx
http://casguns.homestead.com/Sharps_Pepperbox.html
.