Interesting S letter on a Gordian antoninian

[Tibsi]

The die engraver from Antiochia used two joined little C for S.

[dougsmit]

i wonder if his Greek background caused him to cut a lunate sigma first and then correct it into an S.

[Robert_Brenchley]

That would be a terribly small sigma by comparison with the rest of the inscription. Maybe a slip of the engraver, or two goes with a sigma-shaped stamp which didn't quite line up?