That is an older grip, but nothing that can be considered as "rare".
It is not bakelite, even though it has the appearance of bakelite items. It is actually the flakes of woven fiberglass that were not completely covered by the resin. This is not the result of wear, just the manufacturing process then in use. Early no trap buttstocks and handguards also have this appearance to varying degrees. Of the furniture on original rifles I've observed, this was in use on XM16E1 and the earliest (700,XXX-800,XXX range) A1 rifles manufactured by Colt. It also shows up frequently on H&R produced rifles that used the no trap buttstocks. A1 furniture then became the more commonly encountered semi-glossy solid black that did take on a certain shine the more it was used, followed by the later matte black that was actually produced from a stronger material.
Hope this helps,
Wpns Man