Dave, In 1928 the Standing Liberty quarter was stamped with either small "s" or large "S" mintmarks. The large S can look like an inverted small s. That might be what you are seeing with this coin.
Thanks Terry. I was aware of the small and large S mm. I was told by a variety dealer that this could be an inverted "S". I have seen 2 certified examples.
here 2 different mint marks on 2 1928-S STLQ.TL there in the Cherrypicker Guide page#147 4th edition vol II the top one looks inverted like your the 2nd is a large S
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