I found a 1965 Mustang for sale. Looking at the pictures there appears to be red oxide on the underside where paint has worn away? I remembered posts where members discussed red oxide on the underside of the car and it seemed to be a difference of opinion whether red oxide was used or not.
https://www.classic-trader.com/uk/cars/listing/ford/mustang/mustang-289/1965/33677
https://www.classic-trader.com/uk/cars/listing/ford/mustang/mustang-289/1965/33677