Since the 1989 ban imported rifles have had the year of import stamped on them. It was either a two diget number in the model name.
Examples
MAK-90,NHM-90 = imported in 1990
MAK-91,NHM-91 = imported in 1991
BWK-92 = imported in 1992
If imported later than the two digets in the model name it will have a two diget prefix or suffix on the serial number to denote year of import. The x's are the serial number.
Examples of 1991 imported rifle.
91(space)xxxxxx
91-xxxxxxx
91xxxxxx
xxxxxx(space)91
xxxxx-91
xxxxx91
The rifles imported in 1992,1993,1994 will have 92,93,94 in place of the 91 in the above examples.
Just to show there are no rules carved in stone when talking about Chinese rifles I have a MAK-90 rifle that has the serial number 90(space)xxxxxx. But my other MAK-90 rifle doesn't anything but the MAK-90 stamped on it with nothing added to the serial number.
I have a MAK-90 with a 93 serial number prefix so it is a 1993 import. It was new in the box with the rifle and the accesories that were still sealed in the factory plastic bags when I bought it in 1998. Just about every gun show I go to there will be at least one dealer with a brand new in the box Chinese AK rifle. As you know Chinese guns and ammo were banned from being imported in May of 1994.