Your info is wrong. A knee mortar that is unregistered can be destroyed by drilling a bore size hole in it. Then once it is destroyed you can file a form one and reactivate it when approved. It doesn't have to be a papered dewat or any other older style registered deactivated unit. You would be making a new firearm. Only machine guns can't be made with a form one. Unless it was papered before 5/19/86. Everything else is fair game.