The E is known as the Battle E. The slash beneath it means the squadron took the Battle E in two consecutive competitive years. It's a readiness award given to the best squadron and is given by the common superior among the squadrons.
My guess is the S is for Safety. Can't tell from the photos if the outline of the S is green or gray. If it's green, it's Safety. As I recall, the Safety S is awarded to those squadrons who meet the highest safety standards during safety inspections. Be careful taking my word on the Safety S. I'm an old shipdriver and the aviators will do things differently.