Hello and welcome to Just Answer. I'll do my best to sort through all of this for you.
First of all I'm not sure if the IRS has a record of all the tax returns dating back to the 1970's. As fr as I know they only go back about 6 years.
If in this example the man died in 1984 then his estate would have been taxed on the $20,000 back in 1984.
I'm going to opt out of this one though, because there have been many changes to the law since 1976. Maybe one of the estate attorney's may be more knowledgable in this area.
Thanks for trying. It is late in the day to be asking. Maybe someone out there will be familiar with the before and after changes and see the question in the morn.