Quote from Spindrift's linked "American Rifleman" article:
Manufactured in Bayonne, N.J., the Side Loader's receiver is made from a durable proprietary brass-plated alloy called Brasslite, which is treated with a clear coating to preserve its mirror finish. This makes the receiver stronger and lighter than vintage brass options. The barrel band and buttplate share the same high polish, mirror shine.
The brass of my Pedersoli/Navy Arms rolling block is developing a nice patina, also.