This is just a guess, I'm no expert. Perhaps it is a T and F in a circle but it got cut off. Theodor Fahrner had a mark similar to that and did a lot of art nouveau pieces. .
Oh shoot! I just looked at mine again an noticed the s isn't backwards! It's the exact same symbol as yours but looks like this $ Does anyone know what that one is??? .
I am looking for the same mark as well. It's on a silver snake ring and a silver ring with a blue stone. It has a copyright sign beside it and says sterling. If anyone can figure this one out it would be great because i've been looking everywhere .