The synthetic stocks appeared in here in 2009 on rifles imported by "Bell Distributing" (IIRC). My registration certificate lists it as a Poly Industries rather than Norinco, which was better regarded in the US than the Norinco rifles back when they were both available there.
Shortly after the Bell Imports, other distributors like Marstar started to recieve rifles with synthetic stocks and bayonet lugs on the flash suppressors. IIRC, they are still referred to as Norincos.
The synthetic stocks are definitely an improvement over the wood that came on the earlier Norincos. While I've seen some nice looking examples that were stripped down and refinished by their owners, the wood remains very soft and not as durable as the hardwood found on domestically produced rifles. The seemed to work fine, but I picked up some synthetic USGI stocks for my older M305s when someone here on CGN had them for sale. The USGI stocks are fiberglass, lack the stock liner (my terminology might be wrong) found on the both the wood and synthetic Chinese imports and are somewhat narrower with checkering around the grip and forend.
I'd probably rate the USGI fibreglass as more durable and attractive, but since fatter National Match stocks are available in the US for target guns, I'm not sure if the fatter stock has some benefit for the shooter.