As I'm the 3rd owner, I do believe it was more than likely originally black underside. I desire to paint the underside in red oxide for better contrast and ease of cleaning. This is acceptable to Aust. concours judges, despite where it was built and its original colour.
First - welcome John, glad you found the place.
As far as what is being accepted over there we hope (with a little help) that they will join the rest of us on the same page soon so I hate to see you end up with a car that is "dated" but your choice at this point
I have been advised by concours judges here to contact yourselves to seek advise regarding the correct RED OXIDE FINISH AND GLOSS LEVEL for underside chassis. Is there a paint code available? &/or any information to enable correct formulation of paint would be greatly appreciated.
Well if your car was built during the right months and at one of the other two plants that year we would provide the following
The original stuff was an epoxy primer sealer so the finished produce was a very smooth (as opposed to the flat rough finish of many straight red oxide primers) and somewhat glossy to the look. Of course the overspray from the body color and pinch weld black outs will add some dullness where the paint was not applied in a nice coat such as some of the floor pan surfaces further in from the rockers - remember that the rear wheelwells and exposed frame rail there typically got allot of exterior color and really had a nice shinny finish originally
Over hear most of us (where we can still get it) start with a Ditzler epoxy primer that is red oxide in color and tint it from there - of slightly or much more towards the brown side rather than the red side.
Don't forget to reproduce the drips along all the edges of any panel that hung down during the painting. These bumps are a result of too much paint being applied by the apparatus that the body passed over.
Hope this helps a little and good luck with your choice