The law was written that way for a reason. If the current possessor of such a gun were not considered the violator, anyone caught with one would just say "I got it that way". as mentioned, the usual way of dealing with an illegal gun is to call BATFE and arrange to surrender it. I have never heard of an arrest or prosecution where the party has come into possession of the gun in an innocent manner and turned it in.
If it should happen that a smoothbore handgun were received from the factory, I think I would first call the factory. Even today, most factories have good relations with BATFE and it is likely they could arrange to have it returned, not turned in or destroyed. (Colt has every FFL and tax stamp there is, so the gun would be legal for them to possess.)
Jim