My first thought when seeing this was that the letters were FS.
Then I swayed between the first letter being an I or a J as others suggested this. There are stylistic variations in Copperplate and the engraver obviously had to adapt the letters to be decorative and fit within the shape of the locket. I don't think it's an I or a J because there is no tail for the J and there is no large top loop which you normally see on a copperplate I and J.
I am now torn between it being a T or an F. I am not sure if the central flourish is intentionally there to cross the F, or as part of the general decoration.
An example at the bottom of this page (there are thousands of others on the internet):
http://www.rootschat.com/links/01il3/I think it's FS - that's the way I write capital Fs, having been forced to learn copperplate in primary school in the Iron Age 1950s. 
That's the way I write capital Fs too having taught myself copperplate for fun.

Is it a family locket Frank, and if so can you identify an FS or TS who may have owned it?