The housings were probably cad plated. Businesses that do cad plating are under EPA pressure to cease that method. The best substitute is to use dull zinc plating. Dull zinc is a relatively new process, about 7 to 10 years old, and most plating shops have the capability. Consider dull zinc as an alternative.
The housings look to be made from multiple pieces, which may have been plated separately then assembled. If that's the case, there is a possibility that if rivets were use, they were not plated.
Jim