Actually, that watershield is installed backwards. The black water resistent side should be towards the door internals and the paper side should be against the door panel. And by the looks of it, it may not be original (any water on the paper side will leave a stain).
The long held belief as confirmed by many unrestored Mustangs and by Jeff's ginormous gallery of pictures is that after primer, the interior colr was applied prior to the exterior color. Nothing was assembled on the door during painting; however masks were used between the painting steps (masked exterior doors, etc. during interior paint; masked interior painted sheet metal during exterior paint). It is quite possible that the center of the interior part of the door where the door panel would go was not masked, thus some exterior color showing there.
Regards,
Pete Morgan