That's just a 3rd party add-on that the owner apparently never got around to wiring in, it's not part of the original game and is not needed, but if you want to add fast automatic fire it goes between the fire button (using the 'in' and 'out' wires) and main pcb on the harness. Although the speed adjustment potentiometer on it looks questionable with all that rust, you had probably better clean it up with some contact cleaner.