"function or not, if that 80% finished lower has a serial number on it, it is legally a firearm"
Uh.... <sigh>... no.
Depending on specific circumstances. If the manufacturer puts a serial number on it, and lists it as a firearm, whether 80% finished or 0% finished, it is legally, from that point on, a firearm.
If the manufacturer stamps "anything it wants" on it, and does not list is as a firearm, then it is not a firearm, its a lump of aluminium, steel, etc. Once the maker registers it with the ATF as a firearm, (finished or not) it is a firearm.
Prior to May 19th 1986, the makers of full auto firearms registered as many recievers with the ATF as they could, before the cut off date. Many, if not most of these new "firearms" were at the time unfinished. They became legal firearms, "manufactured" before the cut off date, and therefore able to be sold to civilians under the law. The fact that it was days, weeks, or months before the firearms were fully finished was irrelevant. Under the law, they became "pre-ban" because they were registered before the effective date of the ban.
Those people selling partially finished recievers, are not selling "firearms" in the legal sense, because they have not been registered with the ATF as such.
My point is that, if you have a partially finished, or even unfinished reciever, with a serial number on it, it could legally be a firearm. I'm sorry I wasn't as clear as I meant to be. Yes, you can have a partially finished lower, with numbers on it, and it is nothing more than a lump of aluminium. BUT, you can also have a partially finished lower with numbers on it that is legally a firearm.
And without a statement from the maker/or the ATF, for your specific "lump of aluminium" I don't see any way to tell the difference.
One would assume that if the maker sells it mail order, without going through an FFL that it is not a legal firearm. But without some documentation stating such, one can only go on the assumption the seller is complying with the law.
I prefer to assume that if it has a serial number on it, that it is a firearm, legally. Doing so, (and following ALL applicable laws) keeps one out of trouble. After all, if its not legally a firearm, why bother putting a number on it?
As to those partial finished recievers that are not legally firearms when sold, the buyer becomes the manufacturer when the reciever is finished, and assumes the legal responsibility to register the gun with the govt in the approved manner. And yes, there are separate rules for how/when to do this, too.