And the HPA does not supersede the US Code, it is in addition to it.(The title 18 law is still current and on the books.) It provides a method of creating items that would be illegal under the US code by providing for the items to be marked. If the government chooses to they can still enforce the US code title 18 law against unmarked pieces.
As for the items made under the HPA I do find them to be a bit of a gray area. If the word COPY is not in the die or mold, technically all the pieces the company makes would be illegal from the time they are produced until they are counterstamped COPY. And the dies and molds they make them from would also be illegal. I think in these cases the government may be taking the intent of the companies into consideration. Since they know the company intends to mark the items with COPY, they let enforcement against the temporarily unmarked pieces, dies, and/or molds slide.