If the gun wasnt registered it cannot be registered. Even as a 07/02 SOT manufacturer I can take a existing non nfa shotgun and "make" a short barreled shotgun out of it and register it but, I cannot register a nfa weapon that wasnt originally registered. There was an amnesty in 1968 where they allowed people to register weapons in their posession but since then you are out of luck.
If you dont care about having an original, you could have a class 2 build you a SBS from an existing shotgun. Then you could even have it engraved to a 4E or something. If you are capable of building it yourself you can pay the tax and fill out the proper forms and when approved by atf you can build it yourself.
