How the flip would they know if it was complete or in parts at the time of the ban?
I would imagine that if it was sold as complete rifle by the manufacturer, it would be assumed it was a complete rifle at the time the ban was in effect.
If it were just a lower when it was sold, it would probably shift the burden of proof to the owner to show that it had been assembled prior to the ban.
But I don't know that as a fact and I don't remember anyone getting prosecuted for having a built a preban serial numbered receiver into a fully featured rifle post ban.
I do remember some lowers that were found after the ban whose serial number range was earlier than the ban enactment, but the advice at the time was that building a "new Assault Weapon" using those lowers would be against the law.