I received this in change last week. I almost missed the S mintmark as it's the only way to tell it's a proof.
That would be a San Francisco business strike. The U.S. Mint starting in 2012, has sold ATB quarters struck at the San Francisco Mint by the roll. https://catalog.usmint.gov/on/deman..._detailsSAMONA=20ARB&source=detail&Quantity=1