My understanding is that these are perfectly legal, as long as you register it before Jan 1. After that, it would be legal, defined as a part, UNFINISHED, but if you finish it off AFTER Jan1, you cannot register it as an AW, therefore would be in possession of an unregistered AW.
There is NO WRITING in the new law that defines or describes 80%'ers, therefore they would still be legal to purchase after Jan 1, since they are not receivers, and not regulated.
Just my $.02