IMO they are the compression mold cavities.
I don't think Die is the right word.
I think PS house molds are either 8 or 10 cavity molds.

Say they are making a $5 chip:
Red clay composition is molded to the right size with cut outs for the inserts.
Worker put the inserts in by hand.
Inlays are then put on.
After that they are run through the mold cavities and compressed.
If I remember right the H&C mold has 96 cavities.