Or perhaps a counterfeit coin? If you can get that coin authenticated and, it is in fact a real S strike, as Gary Burke said, enjoy the new found cash flow!!!
I have never seen any such thing in any reference book - only Philadelphia made quarters that year.
Counterfeiters have been known to copy date/mint combinations that don't exist. This has been well-documented with Mercury dimes, but I am sure there are others. If these coins really have "S" mint marks, they are likely counterfeit. (I actually like contemporary counterfeits, and I have a few in my collection; they are an often-ignored, but interesting, part of the history of coins.)
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