Pre ban vs Post ban.
Ban items were cosmetic items only that typically had very little to do with the function of the firearm:
Semi-automatic rifles able to accept detachable magazines and two or more of the following:
Semi-automatic pistols with detachable magazines and two or more of the following:
  • Magazine that attaches outside the pistol grip
  • Threaded barrel to attach barrel extender, flash suppressor, handgrip, or suppressor
  • Barrel shroud that can be used as a hand-hold
  • Unloaded weight of 50 oz (1.4 kg) or more
  • A semi-automatic version of an automatic firearm
Semi-automatic shotguns with two or more of the following:
  • Folding or telescoping stock
  • Pistol grip
  • Fixed capacity of more than 5 rounds
  • Detachable magazine
Like I said-very little to do with the funcion of the firearm. There was also a "High Capacity" magazine ban that was a sidesaddle to the AWB ban. Some states still abide by the AWB, with California going above and beyond with their regulations.

922r actually went into effect in 1990, the AWB in 1994 and even though the AWB "sunsetted" 922r still applies.
922 (r)
The Imported Parts Law (1990)
178.39 otherwise known as 922(r) 10 Foreign parts law on semi auto Rifles & Shotguns
http://www.atf.treas.gov/regulations/27cfr178.html
Sec. 178.39 Assembly of semiautomatic rifles or shotguns.
(a) No person shall assemble a semiautomatic rifle or any shotgun using more than 10 of the imported parts listed in paragraph (c) of this section if the assembled firearm is prohibited from importation under section 925(d)(3) as not being particularly suitable for or readily adaptable to sporting purposes.
(b) The provisions of this section shall not apply to:
(1) The assembly of such rifle or shotgun for sale or distribution
by a licensed manufacturer to the United States or any department or agency thereof or to any State or any department, agency, or political subdivision thereof; or
(2) The assembly of such rifle or shotgun for the purposes of
testing or experimentation authorized by the Director under the
provisions of Sec. 178.151; or
(3) The repair of any rifle or shotgun which had been imported into or assembled in the United States prior to November 30, 1990, or the replacement of any part of such firearm.
(c) For purposes of this section, the term imported parts are:

(1) Frames, receivers, receiver castings, forgings or stampings
(2) Barrels
(3) Barrel extensions
(4) Mounting blocks (trunnion)
(5) Muzzle attachments
(6) Bolts
(7) Bolt carriers
(8) Operating rods
(9) Gas pistons
(10) Trigger housings
(11) Triggers
(12) Hammers
(13) Sears
(14) Disconnectors
(15) Buttstock
(16) Pistol grips
(17) Forearms, hand guards
(18) Magazine bodies
(19) Followers
(20) Floor plates

A "Post Ban" weapon does not have to have US parts on it as long as it is not modified in any way. Look at a Norinco MAK 90 in it's original form.
Now if you were to insert a high cap mag into the same MAK 90, according to BATF regs you have modified it and it now must have the compulsory US parts count.
It is perfectly legal to modify a post ban weapon. If it is a foreign made weapon however 922r applies. If it is a domestically made weapon you can disregard 922r.