(e) Any other weapon
The term “any other weapon” means any weapon or device
capable of being concealed on the person from which a shot can be discharged through the energy of an explosive, a pistol or revolver having a barrel with a smooth bore designed or redesigned to fire a fixed shotgun shell, weapons with combination shotgun and rifle barrels 12 inches or more, less than 18 inches in length, from which only a single discharge can be made from either barrel without manual reloading, and shall include any such weapon which may be readily restored to fire. Such term shall not include a pistol or a revolver having a rifled bore, or rifled bores, or weapons designed, made, or intended to be fired from the shoulder and not capable of firing fixed ammunition.
https://www.law.cornell.edu/uscode/text/26/5845The letter already cited this and stated the difference is once it's concealed on the person it becomes an AOW and thus under the purview of the NFA. No concealment, no NFA.
It can't be a handgun because it's over 26" and has a VFG, and it's not an AOW because it's not concealed. But concealing it makes it an AOW because you've demonstrated it is capable of being concealed on the person, and thus an NFA item because of it's configuration.
Now I need a drink. That gave me a headache to type.