It might have been one that was assigned to some type of dealer event. When Ford is announcing a new car they bring 50-200 dealer reps together and host an intro party where they tell them about the car, options, etc. Then the dealers drive home in their car that was there.
This one at Ontario Speedway had over 100 black/gold Mustang II.
Sometimes there were cars built with a promotional tie in to a product - Think pink Mary Kay cars - red/white Mustangs with red and white interiors for Coke (I've seen claims but no proof) - Jeeps with Levi interiors.
It was nothing new my 59 Studebaker Wagon was part of the dealer driveaway intro at the factory. Then the dealer was able to order his car and pick it up at the event where they toured the plant and watched the new Larks being built. Mine was ordered with every option checked. I figure the dealer got it for his wife.