My understanding, and this is decades old is that fibrite was made from a long fiber polymer. Colt was experimenting with various materials for the M16's furniture and experimented with acetate covered aluminum as found in the CAR 15 telescopic stock and Bakelite in the pistol grip. Fibrite wasn't as strong as required and was eventually replaced by foam filled nylon. There's lots of good info here: Variations in Early AR-15/M16 Rifle Buttstocks - Armament Research Services (ARES)