There were quite a few fiberglass stocks on the market in 1980. Chet Brown of Brown Precision introduced fiberglass stocks in 1968, and after a few years there were some imitators. My first fiberglass stock was purchased from Chet Brown back when he was in San Jose, then I tried a few from Ultralight, then one from a maker in the midwest somewhere. Finally found Lee Six and bought stocks from him until he retired a couple years back, his stocks are available from Kelbly's now. Most use a polyurethane foam as filler, not Styrofoam. Lee Six used an epoxy foam that really made the stock rigid. Not the lightest stock, but very good.
McMillan stocks have always been injected foam with glass fibers, similar to the HS Precision stocks only without the aluminum blocks.