From a copy of The Lost Bullet, here is the Hughes digitized film and I can definitely see something move into position in the sniper's nest window which corresponds with the position in your 3d graphic.
The following graphic suffers from the usual gif restrictions but even in this copy from a copy from a copy, movement can be seen.
@ about 40:00
JohnM
Another great post John, thanks. Both Holland and Dale Myers believe that the sniper was standing for the first shot due to the sharp angle and subsequent box interference with the view. I believe that it is more likely that he was sitting on the box on the floor for all three shots. And that without any practice shots, the sniper didn't realize the boxes would interfere (until they did). And I believe
that is the most likely explanation for missing the entire limo from that angle and distance (not hitting the traffic signal).
Here are a couple of images showing where the sniper would have to be positioned (in the little cubbyhole between the stacks of boxes) in order to have an unobstructed view through the window opening of the limo, at that point in time, if he were standing.
That space is a tight squeeze with no room for maneuvering. Not a likely spot to fire from in my opinion.