And after you discover hundreds of John Smiths who died between 1980 and 2030 and many more who are still alive, what then?
Depending upon the profile of the work in question, that may be difficult or impossible. If the work in question was famous and gets mentioned in the author's obituary, things will be simple. But if it's a small vanity-published item that never achieved much notice, there may be no trace of it anywhere.
BTW, another thing I was curious about: suppose someone buys some positive reversal movie film (camera original) at an estate sale. Would the purchase of the film constitute purchase of the full rights thereunto? What about manuscripts, photographs, computer media, or other such materials? (I mentioned movie film first, btw, because it is the type of media most likely to be a one-of-a-kind item).