I would wager that there are lots and lots of frames and receivers as well as kits owned by people but not yet assembled.
One of my fears from reading the article, is that the ATF might want to return to the good old days of firearm manufacturing. What I mean by that is that it use to be that many rifle parts or pistol parts would have serial numbers or at least the last 4 or 5 digits on several parts to show that the firearm had been properly hand fitted.
If in say an AR, one needed to have a matching set of serial numbers on the lower, on the bolt carrier group, on the upper, the barrel and maybe the buffer tube, then that would be time consuming and costly. To say nothing of creating a nightmare when changing parts out.
P.S. The government, under the Bill of Rights is suppose to pay for the taking of private goods. If implemented, the ATF could be on the hook for a lot of products they effectively took by making illegal. Of course the Supreme Court could be stacked so that the Bill of Rights is meaningless.
I’ve seen older pistols and battle rifles with major components marked with partial matching s/n, but I assumed this wasnt a gov requirement, but due to hand fitting required for those older models within the factory to help avoid part mixup.