It's either Cad plated or galvanized, sort of a dull silver gray finish. For mine, I bead blasted the housings to rough them up and had them clear zinc plated, it's about the only practical finish that's available in California. On daily driver class cars, I spray paint the inside with Eastwood chrome paint to make the lights brighter. A normal zinc plating process does not plate well on inside surfaces. There is rubber insulation on the four studs and the rubber seems to handle the acid used in the plating process. The light socket can be pressed out and back in easily.
Jim