Vague-er-er than that, even. What new 80% makers go off of if they don't get explicit approval (and ongoing supplementary approval as well, apparently) is the
expectation the ATF will make a similar determination as what they have done in the past. In the case of AR lowers, the ATF made dozens, and dozens, and dozens (and probably more dozens) of nearly-identical determinations over the years approving designs with:
"1. Leaving the fire control group pocket (including the associated holes) solid.
2. Leaving the magazine well solid.
3. Leaving the buffer tube hole undrilled/unthreaded."
And requesting that submissions which did
not receive approval include them as well (and then approving them). It's hard to argue that doesn't establish precedent, because it most certainly does, but because the ATF is not a court of law, that really doesn't amount to a hill of beans at the end of the day. The expectation of their opinion remaining the same on lowers was nothing more than that.
I believe there are also rapid-prototyped lowers out there which utilize two-color plastics to denote the areas to be removed. Unlike the EP lowers, those two materials truly
are identical and differ only in color; EP has a fiber reinforced wrapper around a solid plastic core section. ATF probably isn't pursuing those because they don't want to have to be the ones to prove in court they can be readily made into functional firearms
(RP isn't the most dimensionally consistent stuff, and you'd have a dickens of a time removing just the off-color plastic and expecting the gun to work)
Another fun fact; the EP core has ridges cut into it which key it to the pocket walls of the "lower" as it is cast around it (defeating the ATF argument on its face). Moreover, this means that merely removing the "self-guiding" 80%'s filler material is not enough to complete the firearm; those ridges must be removed back to a measured position and the holes precisely drilled as well. That really does make the operation not hugely different from completing an Aluminum 80% by following a scribed or marked line with a manual mill. In the past, tooling and material hardness have not been criteria for determining whether a receiver is complete (I've only seen that line of thought applied toward determining whether a semi-auto system can be readily converted to full auto; a legal AR15 receiver can be converted to an M16 about as easier than one of these lowers can be finished out, fwiw, and an AK even faster)
TCB