My own personal belief is that during original production, the inside of the clamshell doors and landing gear doors were never actually painted silver, but left in natural aluminum (with the clamshell doors either having bare stainless rub plates, or chromate yellow rub plates, earlier in production). This is the first time I have seen a color view of the clamshell doors and landing gear doors, of WWII-period, with them being anything other than the silver-look of natural aluminum. The wings were profiled and painted silver, but the exterior skins of the landing gear doors, clamshell doors, flaps and ailerons, and the large wing fuel tank panels, were left natural aluminum. You can see the bright marks of the brushed-on acid application on the landing gear doors seen on all of the mustangs within the shot. As copied on the restored Happy Jack's Go Buggy, Little Rebel, and Upupa Epops, in order to prepare the metal before spot-welding was to occur, they would simply brush on acid, which would etch the metal, and leave behind these variations in the metal in different areas of the aircraft where spot-welding took place.
Just for kicks, here are some more screen-captures from the old "Great Planes" documentary on the Mustang, which is still one of, if not the best ever made on the aircraft.
In this first view, as the P-51D taxies out, you can see this P-51H waiting in line for take-off:

At this point, the aircraft's final finish/markings application had not yet been applied, but the wings had already been profiled. I've always been curious whether or not the chromate yellow panel on the side of the aircraft would have been painted over with silver, or if the chromate yellow would have been stripped off before the aircraft was ready for delivery (NAA was quite particular about the finish on their airplanes)



Views from the cockpit of a P-51D-30-NA, as the others are, shown within the original period footage from the Inglewood plant.



Final finish/markings not yet applied, but the wings are already profiled.





An example with the final finish/markings applied, with the serial number clearly showing it as a P-51D-30-NA (besides other noticable details).

