I think you have a banker's mark, or a test mark to check for a plated or low silver content coin. See a similar S on the example below from CNG. The quote below is from a CNG description of this coin. Mark Antony minted these coins from silver he received in Cleopatra's dowry, but there wasn't enough to pay his troops! So these coins were minted at a slightly lower standard than other Roman issues of the time. This sent Greek bankers into a frenzy, and for a time they tested all Roman silver. Antony's denarii nearly always have a shallow "banker's mark", while Octavian's and other issues of Antony that were tested often show a deeper punch. The reason? Pure silver is softer. And why are these legionary denarii so hard to find in high grade? Bad money replaces good; these legionary denarii circulated into the reign of Gallienus! Thank you, Harvey Hofer, for this (true) story. (Remembered from circa 1961 AD).
We always say these are Banker's marks but I am unclear on exactly what that means. It could be a mark that the coin was examined and found good but that could be an assumption. I could also be a mark of ownership so a thief might not want to be found with a bag full of S marked coins. There are many different style marks. S is not rare. Meaning? I have no idea.
@Andres2- that's just because "S" stands for Sestertius. And that one was also minted in San Francisco.
I think it stands for Sabina , Hadrian's wife. He said "" its that she is an Augusta otherwise I would divorce her immediately""
