I don't like it, but I do understand why ATF claims a plate is a gun. Without drilling any of the rivet holes, you can probably take a standard 80% plate, slip it in, put the top cover bolt through, clamp the rest of the plate in place, and probably fire at least one shot. The thing may blow apart on you with that one shot, but in the eyes of ATF, it fired so it's a gun. Pretty freaking ridiculous but that's how they will approach it.
I think 80% can be sold by leaving the plate a little bit more in "unfinished" state. For example, I'd say perhaps, machine the cocking slot but not all the way through, so that the builder will have to finish cutting the slot through to the other side. If it was sold like that, ATF could not just slap it on a 1919and let off a shot, it would require "machine work" of some sort to finish the plate to usable status.