Steve; you are probably right!
My 1960 shop manual doesn't show it in the headlight circuit diagram 13-4, but my 1959 shop manual shows it on page 11-6 fig.1 Headlight circuit Diagram.
Your truck may have been on the cutting of ford electrical technology. Just out of curiosity where your wiring goes through the fire wall is it round or square? (I think 60's were square)

 

Mine is factory. The book say's internal. That should have been one of the first things i checked. I can however claim my glasses are old