There are two versions of the NHM90/91, both made by Norinco. The first is the NHM90/91(AK47 Thumbhole) imported in 1991-1993. Came with a 5 round mag, in 5.56x45 or 7.62x39. This, apparently was the original rifle referred to as the Mak-90.
The second NHM90/91Sport had a choice of 16.34" barrel (NHM-90) or 23.27" barrel (NHM-91. The NHM-91 came with a bipod, and a hardwood thumbhole stock.
I have an NHM-91 Sport. The stock is varnished wood, with a funky recoil pad. It has the longer barrel and bipod, and what appears to be more of an RPK contour barrel, chrome lined. When I bought it, only the mags designed for it would fit without modification. There was a rivet installed in the mag-well, and the magazine was machined with a groove in the locking lug, to allow the rivet to pass.
Fortunately, the rivet soon loosened, blocking any magazine insertion, and then fell out. Today, the gun accepts any AK magazine.
I don't know why the Chinese would have bought technology from Steyr for this rifle. They were quite adept at stealing and copying anything that they found interesting. I do believe that the story was just another "I heard" selling point.
The rifle is accurate, and has the nicest trigger that I've ever shot on a Chinese rifle. Accuracy may well be from the longer barrel, and sight radius, rather than any barrel technology, though.
According to the Blue Book, a 98%, pre 1994 NHM90/91 is worth $675.00. The later NHM90/91 is worth $500.00.
Amazingly, I bought mine, NIB, with the 3 5-shot mags, sling, and cleaning kit, in the mid-1990 time frame, for $275.00.
I'm not familiar with any having a painted stock, all of them that I've seen were thumbholes, with varnished wood. A group of County Police that I shot with back then all bought the "Mak-90" versions, and cut and shaped the thumbhole stock into servicable versions of the original AK wood.