Innaday, most of the Ford products had some sort of underbody paint finish, after the wash and bonderite chemical dip. Well preserved cars show a red oxide of sorts, but there is the urban legend (repeated by my father, a 39-year FoMoCo engineering vet) that it was a slurry of paint left over from the tanks of individual colors at the end of the day. Would like to hear from a plant vet who was there to confirm/deny.
If your car was undercoated, it would get hosed down with a semi-flat black finish that would dry firm, but not hard, so it could self-heal from rock chips, etc. This was very common in the midwest and northeast, especially metro Detroit which is positioned directly over a salt mine.