selector filled usgi fiberglass stock.
springfield armory used to fill the selector hole on surplus usgi stocks, paint em with a textured paint and sell em on thier M1A's . these days i think they are a resin injected stock much like the M305 stocks, just a little nicer
there are also a crap load of cgners with this mod done to usgi stocks and they do pop up from time to time in the EE.