There are three main classifications of AK stocks, all of which are influenced by U.S. law.  The first classification of AK stock sets are imported pre-ban pistol grip versions.  These stock sets were issued to pre-89 ban AKs (as pictured above) that were imported into the United States. The stock sets can be made of wood, polymer, metal and can fold or retract.  For example: Polytech Legend, Styer Maadi, Hungarian SA-85M, Norinco rifles, and Yugo imports.  All of these rifles incorporate pre-ban pistol grip versions of the stock sets.  Some of which are issued with folding or retractable stocks.  Pre-89 ban rifles do not have any restrictions on what type of stock set you put on it.  However, make sure that you do have a pre-89 ban AK or you will be in violation of U.S. law. However some post-89 ban AKs were issued with pre-ban style stock sets.  This is because the ban only applied to imported weapons, thus domestically made AKs were still allowed to have pistol grip stocks.  It was not until the 1994 ban that prohibited pre-ban style stocks sets on domestically made AKs.  

The second classification of stock sets are post-ban sporterized versions (as seen to the right). These stocks were issued to post-89 ban and up imported weapons.  It wasn't until 1994 that they were issued to domestically produced AKs after the 1994 ban.  The U.S. government believes that these stock sets make the gun less deadly and more appropriate for civilian use.  After the 1989 ban, imported firearms were not allowed to have more than one feature on a list that were considered inappropriate for civilian use.  In 1994 domestically made firearms fell under the same law, except they were allowed two bad features.  This stock set was designed to allow the importation of AKs after the 1989 ban and successfully bypassed the governments laws.  However the guns that did have this stock also did not have the following items: bayonet lug, pistol grip, flash hider, folding stock or a threaded muzzle.  It was and still is illegal to add one of these items to a rifle that falls under this law, unless you replace one bad feature with another. As a result a new classification of stocks sets were produced to bypass this law and allow people to have two bad features on their post-ban rifle. 

The third classification of AK stock sets are U.S. made post ban versions.  These stock sets were produced to bypass the 1989 and 1994 bans, which prohibited pistol grips on post-ban imported AKs. The stock sets are made in the U.S. to comply with section 922(r) of the ban.  In conjunction with other U.S. made parts the stock sets allowed people to put a pistol grip on their imported post-ban AK.  This is done by reducing the total number of imported parts on the weapon, reclassifying it as U.S. made.  Thus people were allowed two bad features on their AKs, giving the rifle back its military look. This became very popular with firearms owners, giving birth to a new wave of pre-ban style rifles.  Today there are over seven different manufacturers of U.S. part kits.  U.S. stock set versions now come in the following configurations wood, polymer, metal and pinned folding versions.  A pinned folding version allows the owner to give their rifle the pre-ban folding look while remaining legal.  

Below are some other versions of various stock sets which the AK was issued with.  They each have different classifications and legalities that accompany them.          

Under Folding

Para-Trooper

Pistol Handguard

Polymer Fixed

AK-47.net dose not claim to be able to interpret the law, you should check with your local and State authorities concerning the regulations of these rifles.