What is it exactly? I see one on the ejection seat and two near the canopy on the left side.

If you only see two in the stowage on the left hand side of the cockpit coaming ........... Bad boy ! you failed to check before you started your take off run (as per FRC's).

If you forget the pins the seat won't fire

In the RW on the line the seat is Safe for Parking ,so after strapping in the pilot signals the ground crewman to remove the face-blind safety pin( he can't reach it himself) and he removes the seat pan pin, both are the stowed in the holder with the other two pins.

If the seat is to be rendered Safe for Servicing all four pins are inserted into the various positions that stop the various cartridges used in ejection from being fired .