The reciever itself has to be destroyed in a ATF approved manner.
I respectfully disagree. Yes the reciever is what is registered. But if it hasn't been registered, with the proper barrel, and no proof of the illegal barrel, they have no proof it was ever a cut down SBS. If this shotgun has been registered, there will be evidence on the reciever (engraved manufacturer and address). If that's the case, keep looking for the tax stamp.
If his shotgun has not been registered a SBS, AND the barrel is exchangable, remove the barrel and install a legal barrel. It has not been a SBS (reg.'ed by law) so it could be returned to standard, by installing a new barrel and destroying the short barrel.
If you want to register it, do as stated in the previous paragraph. But instead of destroying the barrel, sell it to a friend ($1 will work, just have paper work to verify), to lock in his safe that you have no access to it. (This won't work if your friend has the same model shotgun. A barrel with no reciever is not a firearm, thus not illegal.) File the paper work for the SBS, when it comes back, buy the barrel back from your friend.
If it has been registered, and you can't find the tax stamp. Well it might be worth a try to contact the ATF, but I would just destroy it. And this is when you would have to destroy the reciever. The ATF has had a way of "losing" the paperwork.
Wyman