A '51 truck from the Chevy factory would have only one tail/stop light. The shop that installed the fire apparatus most likely installed the right light so that would make it right for that truck.
From what I can tell by the picture it appears the lower lights are the stop and tail. The uppers look like after-market turn lights. If the truck doesn't now have turn signals they may have been used for flashing lights for emergency runs. Looks like a good place to wire the signals into.
