You have one of the ultra rare, super secret spy quarters given out to CIA
operatives to provide identification to one another out in the field.
A government representative will be visiting you soon.
Depends on how big an S it is. If it's the same size as a P or D mint
mark, you may have a proof coin that someone broke out of a proof set
and spent. Though "rare" to find in circulation, they're out there.
Some RCC-ers are building a collection of proof coin pulled from
circulation.
If it's larger than a normal size mint mark, someone modified or added
it, meaning it has no collector value, per se.
Jerry