Re: Stevens auto shot .410 pistol
Alpo and Rjay are knowledgeable members. You can usually rely on what they say.
Federal agencies like the BATF, the FAA and the DOT administer and enforce Federal Laws within their jurisdictions. They also have to power to (and often do) make administrative rulings which have the power of law unless overturned in a Federal Court, which happens only rarely.
The BATF has made its share of these administrative rulings over the years. Sometimes they appear to make rulings that reverse previous ones. Sometimes they make rulings that suddenly outlaw legally possessed things. But, they have placed some NFA items into the curio and relic class, also. I do not know or opine as to what the case might be with an antique, and collector item like you describe. Many of the BATF agents are quite knowledgeable on the finer points of their agency's positions, some do not always answer citizen questions correctly.
The type of item mentioned by the original poster was likely removed from the $200 Transfer Tax List and placed on a $5 Transfer Tax List around 1968. Also there was a 30 day amesty in November 1968 when many contraband NFA items were registered and made legal without payment of the applicable Federal Tax. If it was (for lack of a better term) "registered" to the previous deceased owner there may be some BATF or Treasury Dept. paperwork in the estate. In such case it might still be transferable. If the decease owner held it as contraband it will likely remain contraband subject to forfeiture and criminal prosecution if apprehended in possession of it.
I am not current on present BATF NFA positions. This is not my strong field. In fact I stay away from NFA items like I do agitated rattlesnakes. There are a number of collectors of this kind of material who might be better able to advise you as to your legal options relative to it.