Richard,
I'm not sure I can post the pictures I've found in the format I would like, so I decided to drop them in a pdf with the photos aligned in time. Hopefully this will display and additionally allow some direct zooming in on your screen.
https://drive.google.com/file/d/1tsj1RnCbMm5rbo_JtdcAc7RY-M8jVttT/view?usp=sharingI dug around and found versions of the photos that do not readily show the mark, and aligned beside versions of the ones that do. It appears to me to be a function of the resolution and lighting of the photos. If its a mark, I think this would suggest a very shallow surface mark that needs good lighting to visually bring out.
I'm not totally sure on the source of the last couple of sets. I think the last set, with the jump seats moved may be from the FBI or Secret Service. I'm pretty sure the next to last set are FBI versions. One of those appears to perhaps have an inspector with a lab coat in the front seat. I wonder if this could be Frazier looking at the windshield.