I went through my late Dad's coin collection and came across two dimes that I can't seem to find listed. They are a 1956S and 1964S. Attached are scans of each...there is a clear as day "JS" under each head. In the scan, you can make out a mint stamp...but perhaps not the "JS"...but trust me they are as clear as day.
Only 1956 and 1964 plain and D are listed in the coin directories...not an "S".
The "JS" is the designers initials, John Sinnock
The mint mark on those year coins is on the reverse to the lower left of the torch.
There were no "S" mint made in those years.