For general Chinese info...
With the Chinese commercial guns (that is, those guns produced for and sold to the US market) there are many, many different variations. For example:
-Milled rifles, Pre-ban (Polytech Legend, Polytech Legend National Match)386 factory
-Milled, post-ban (MAK 90 Milled, MAK91)
-pre-ban stamped (Polytech Legend stamped, 56-s (semi-auto) 56-s1, 56s-2, 56s-3, 56-s4, 56-s5(?) these were fixed stock, underfolder, side folder, polymer furniture, uh... what else?
Moving on...
So with Chinese guns, you have stamped, milled, pre-ban, post-ban.
Stamped vs milled is pretty obvious. Pre-ban means the 1989 Assault Weapons ban, so guns imported before the 1989 AW ban have all the "evil" baby-killing features such as cleaning rod, muzzle device, bayonet lug, pistol grip, full stock, etc. Post-ban guns were imported after 1989 AW ban and were "neutered" with bayonet lug ground off, no cleaning rod, no "fangs" under the front sight block (which hold the cleaning rod/bayonet, thumbhole stock, etc. To complicate things, guns caught/held up in customs during this time were required to be modified before sale so they will have "evil" model names stamped out, ground off, milled off, overstamped, or otherwise changed. These are referred to as "transitional" or "caught in customs" guns and some may have one or more of the original evil features present. Or not.
Any gun marked MAK (90/91) is Modified AK and was imported after 1989 and will have neutered features. MAK90 may be found in 7.62x39 or .223/5.56mm versions, as well as both milled & stamped versions.
MAK 91 is a milled, long barreled, post 1989 ban version of the Polytech Legend National Match.
The milled Chinese guns are close to type 3 AKs, but with the full auto features neutered for the commercial US market. The stamped Chinese guns are technically AKMs but the Chinese used a mix of milled type parts and stamped type receivers. Stamped Chinese receivers are thicker than other country receivers, excepy Yugo. Typically, Chinese barrels are thicker as well as the barrel components, and wont fit on other AKM barrels.
NHM91 is a stamped, long barreled gun (not a true RPK) but came with a bipod anyway. It has a shortened bolt, a stepped hammer and had a rivet installed in the mag well area to prevent use of normal std AK mags.
Many, many post-ban rifles came with thumbhole stocks, either Boyd's, or Berry's, or an ATI polymer "Dragunov" style. A lot of people throw away the TH stock and replace it with std furniture.
So, to review:
Pre-ban, Post-ban, milled, stamped, fixed, UF, SF, polymer, detachable bayonet, underfolder spiker bayonet, type one bayonet, type two bayonet, 7.62x39, .223/5.56mm, Canadian import, transitional, B-West import vs B-West made receivers, Maadi + Chinese versions...
And I'm just getting started! Chinese guns are fun. I like em.