Bolt and bolt carrier interchange.
OK, here is a parallel. The AR-180 is prohibited. The AR-180B is not. The upper receivers are interchangeable. The lower receivers (i.e. the legal receivers) are different, and the 180B uses a different trigger mechanism. So, the AR-180B is a different rifle.
The Tantal looks like a duck, walks like a duck, quacks like a duck, but isn't a duck, because its receiver is not an AKS-74 (named AK variant) receiver. Its receiver is a purpose built, US made receiver unique to this rifle.
Maybe this is the logic. When the FRT entry becomes available, perhaps things will be clarified.
Still doesn't explain how some firearms are deemed to be prohibited when all they share with a prohibited firearm is appearance.