Stock on your truck would be 3 terminals. High beams, low beams, and parking lights.
4 terminal strips could be used for turn signals (dual filament bulbs in the parking lights.)
Don't know if some (later?) trucks came with 4-terminal strips.
[on edit] Apparently 54-55 1st came with one of each. Don't know what the extra terminal was for.
Link[on edit again] Found a '54 wiring diagram. the 4th terminal was for the horn. (mounted behind the grille?)
