As most people know, the factory never completely painted the under side of these cars.(body color) The bodies were dipped in a gray/black primer.(the color varied by plant)The exterior was then sprayed with a red oxide primer. I'm not sure if they sprayed the body color directly over the red primer or if there was a gray primer sprayed on top first.
The under side would have gotten red oxide and body color overspray. Depending on how far the painter wanted to bend over, determined how much or how little overspray was present. Every car had slight variations in the overspray. These cars were painted by people...not by robots like today.
The biggest mistake I made on my car was telling the body shop to completely paint the underside body color. I really wish I would have gone the primer/overspray route. Even though I'm going the Pro Touring route with my Dart, it will be receiving the primer/overspray look.