Jazz,
That's most likely a foundry mark, not the hammer mfgr's mark, imo. Sivyer (in Chicago) used a Diamond-S. Symington (in Rochester, NY) used a circle-S, but the circle and the S were double-lines. I don't know if the National Foundry and Trademarks list was on your search, but no other symbol on that list looks like your circle-S. There were thousands of foundries. Could be a tough one. Good luck.
Starrett used a circle-S on their tools in their primitive era. I don't know if Starrett was making hammers though.