Tom R,
Good for DAD and his little light bulb. As you mentioned, all the wiring diagrams show only two wires, and the key switch functioning as a ON/OFF switch..........but I sure wonder what that third terminal is for.
I can't believe it is dead. I would guess there is continuity in some configuration your father might not have tested.
Would they have hard wired a dealer installed radio in 1948 so it could be turned on without a turn of the key. Maybe that extra terminal is a prehistoric ACC position for a radio or foot warmer!
Who knows!
Stuart
