The receiver covers on Henry's economy models (those which are not brass) are made of an alloy called Zamak-5: For the doubters here's an article about the Zamak alloy. The receiver covers of the Henry H001 series and Golden Boys are Zamak-5:
Andy Wickstrom of Henry subcontractor Henry Wisconsin which produces the receiver covers and bands had this to say about their Zamak alloy:
"First, the metallurgical standard for this metal is Zamak 5. Inside of the Zamak 5 spec we maintain even tighter control of certain elements to further improve physical properties. Second, the metal is cast using the ultimate in high pressure die casting equipment. The power and speed of this process is impressive. The process used to control the quality of the product consists of countless variables that are finely tuned to produce the very best physical and cosmetic properties."
In addition, HRA has not been using any plastic parts for the last several years.