Fine, a rephrase...
If you build a semi and sell it through an FFL, it is registered. Dont give me any of this bullshit that a 4473 isnt registration, because it is, no matter what the Fed says.
Now, what Im saying, is that if you "all of a sudden" bring-out a non-serialised receiver that has never been on any paper (which would confirm a date of manufacture) you are screwed and it WILL be considered post-ban, no matter if it really was pre-ban, because you have absolutely no way of proving pre-ban status.
Sure, a private citizen (non FFL) can legally make a semiauto receiver, but unless it was documented via a serial number and sold before the ban, it will always be a post ban.
You could have built it yesterday and stamped a date of "1993" on it for all they know.