I think that the "red oxide" is often mistaken as being "correct" for all 65-66 cars, because most of the 65-66 GT350's that may be "well photographed" are restored this way. They are all San Jose built cars though and most (can be exceptions to the rule) are pink or salmon color underneath. Even most of these cars with original paint that I have observed are rather glossy.
First great looking video
Would agree that the original finish was a fairly shinny finish due to the use of epoxy primer sealers rather than the flat primers.
Have not seen any pink or "salmon" colored 64-66 San Jose undercarriages (thousands of pictures and a big bag of sections cut from the floors of San Jose cars as examples
often white overspray over the brownish red oxide can produce a chalky lighter color is areas but have not see the pinkish till late 67 and or 68 at San Jose during some months
We can probably go further back to October of 64 since mine has evidence of the black undercarriage...
Would have to disagree the period of time that Charles is describing is surrounded by many hundreds of well documented red oxide cars
Did you cut through the paint drips to see if the original color was black or if it was black over red oxide. Not unusual at all for a prior owner during the 70-80's to repaint the floors black to clean them up
IF it was black all the way to the metal then we may have another period when black was used but I've not found any San Jose cars from that period with black undercarriages as of yet