fairly certain they are looking at the provenance of the known population of 1964 SMS
Kennedy half dollars which is 12 specimens all of which are accounted for and graded PCGS all came from dealer Lester Merkin who claimed he got it all at the mint director Eva Adams estate sale.
in fact, they are so few and the grading company so confident in identifying them, if one were to be spent or stolen, and turn up to be graded they would know which one it is of the 12.
it's more than a single marker or two markers, they also have to be strongly struck and satin finish and individual identifiers beyond that for each piece.
They never got out into circulation, they were trial pieces for the SMS coins for the following year and all kept by the mint director and bought by Lester Merkin at the estate sale. Chance of finding one is nil. The chance of buying one, much better if it could be afforded they start around $90K at auction when they come up. I wish I would have paid attention better and bought one in 2006-2009 when they were, relatively speaking, cheap still.