From:
http://www.ar15.com/content/articles/primer/
5. But I Want A Pre-Ban Rifle
IT IS VERY IMPORTANT TO NOTE THAT A LOWER RECEIVER WITH SERIAL NUMBER WHICH DENOTES MANUFACTURING PRIOR TO SEPTEMBER 1994 DOES NOT NECESSARILY PERMIT YOU TO LEGALLY BUILD A RIFLE WITH PRE-BAN FEATURES!
A LOWER RECEIVER MUST HAVE BEEN ASSEMBLED INTO A COMPLETE RIFLE PRIOR TO THE SEPTEMBER 1994 BAN AND THE MANUFACTURER MUST BE ABLE TO DOCUMENT THAT ASSEMBLY TO THE BATF IN ORDER TO QUALIFY AS A PRE-BAN LOWER RECEIVER!
This is an extremely important detail! This is why I highly suggest that when dealing with any "Pre-Ban" lower receivers, check the serial number with the manufacturer to confirm that it is indeed of "Pre-Ban" manufacture. Getting this confirmation in writing (i.e. by facsimile) is also not a bad idea!
So, you want all the goodies on your rifle? Well, you can still have them but it will cost you. Assembled Pre-Ban rifles sell for about 40-50% more than Post-Ban ones. So a rifle that costs about $1,000.00 retail, will set you back—if you can find one—closer to $1,400.00. That’s not a lot of money to some people who "have to have it all". With a Pre-Ban Rifle, you will then have the legal capacity to have: detachable magazine, pistol grip, threaded barrel and flash hider, collapsible stock, bayonet lug (bayonet sold separately), and grenade launcher (sold separately).