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This stands for the rating of "Stoker" and goes back to the days when ships' boilers were coal-fired and thus "stoked" by stokers. In the Royal Navy they are now known as Marine Engineering...
Firecontrol Technician second class. An FT2 is an E5 in rank and is responsible for the guidance systems for all large weapons onboard the ship up to and including missles.
This is called a "rating" in the Navy. The AT denotes that person's rate, or job. In this case, it is an Aviation Technician. The 3 denotes that person's rank, which in this case, is a 3rd Class...