I ran into this situation a few years ago while buying a Reising. The guy who had it had found the registered weapon in his fathers gun safe. Father was a collector and had passed away a couple years prior. The son never knew that dad had the SMG which was registered to the dad in 1975. The family started going through the safe gradually selling off items as the father did not leave any instructions in the will. They then found this SMG and were not sure exactly what they had or what to do with it.
In order to buy the weapon, we first had to get his attorney involved and get the weapon legally transfered to the sons possession. That required about a year of paperwork, death certificates, affidavits, letters, phone calls and the ATF registration forms. Once it was registered to the son, I was able to complete my paperwork and buy it from him. All totaled it took about 2 years for me to get the weapon.
Had the father left the weapon to the son as part of the estate, we were told that all that would have been necessary was to do the ATF transfer paperwork and include a copy of the will. As it was, the guy had an unregistered NFA weapon in his possesion and really had no idea of what to do. He was just going to sell it to me like a regular firearm when I first spoke with him. It was a learning experience and I've since indicated in my will what I want done with all my NFA stuff and who I want in charge of making sure it gets done