https://www.ebay.com/itm/305515583429Pretty sure this
Mercury dime is a strong candidate for an inverted knob-S mint mark, FS-504. I had been watching the coin with the intent to buy it, resubmit for variety attribution, and then liquidate. Alas, the price got too high for my risk tolerance. Probably the bidders really liked the colorful toning on this one. I didn't mention it here before the auction finished, so as to not disrupt any CCF bidders.
If any of you bought that coin, you should definitely examine the mint mark under magnification. The coin could be worth quite a bit more than $75.