Yes, it had to have been a complete rifle*, capable of semi-automatic fire, and in AW configuration, and it had to have been legally possessed on the date of the ban. And, you have to be able to PROVE that it was. You can be charged simply based on the fact that you possessed a gun in AW configuration. It is up to you (the owner) to prove that the gun meets the criteria to be grandfathered under the law. Of course, if the prosecutor thinks you'll be able to show such proof, he'll never prosecute in the first place...
*It could have been disassembled, but it must have been part of a *complete* kit, enough so that if it had been assembled, it would have met the description above.
It doesn't matter who built up the gun, as long as it was complete and in AW configuration. But, in most cases it's easier to obtain proof about a factory-built rifle than from a gunstore's build. Just an aside: few gun stores build ARs, as doing so subjects them to paying the 11% excise tax, and dealing with that paperwork.
-Troy