The 1964 SMS were found in the estate of a former mint director in the late 1980s. They were sold to a prominent dealer, and shortly thereafter, slabbed by a TPG. In order for it to be an SMS, somebody would have to brake it out of the slab after paying a significant premium to buy it) and spend it at face value. Then it needed to circulate enough to be AU before you found it. That's about as likely as @GDJMSP giving this post a like