I am confused. Their original ruling was that a shoulder stock pistol was a short barrel rifle and was illegal to possess if not registered as such. That was pretty much in conformity to the original 1938 law. When the law was changed to give BATFE more power to make changes, specifically to define curios and relics, they ruled that a pistol with a shoulder stock was no longer an SBR and thus under the NFA, but a C&R item and no longer under the NFA IF the shoulder stock was original. They did not require that it be original to the gun, only that it be of about the same time and the same type originally issued with the gun.
So far, that has satisfied the antis in Congress while allowing legitimate collectors to have original guns with contemporary stocks.
Perhaps I am paranoid, but I am afraid that if the gates are opened too wide, so that Glocks, say, can be fitted with shoulder stocks, the gun ban gang will set up a dozen mass killings with shoulder stock pistols, and we will be in deep doodoo again.