They were made using RIM molding (Reaction Injection Molding). This process allows two or more componants to be mixed into the injection mold simultaneously under heat and presure, the componants then catalyze into a solid in the mold. It's a plastic product but not a thermoplastic like many other automotive products. The basic main componants are plastic resin, very fine glass fiber, and talc powder. When comingled this gives you a light, stiff, warp & heat resistantant product. When you drill holes in it the talc powder shows itself as a fine white dust. Many confuse it for fiberglass which it is not. Yes, all parts I've seen made in this way for a Mopar had a part# and pentastar molded in.

Here's a short list of some of the other 1960-1976 Mopar products made using RIM molding:

Heater boxes (all years)
Under Hood Air Grabber/Ramcharger Boxes (all years)
A-body dual and single hood scoops (all years)
B-body dual hood scoops (all years)
B-body Door & Quarter Scoops (all years)
Challenger Side Scoops (1971)
1970 Cuda Rocker Molding Gills
Shaker Bubbles