John,
Are you still working for the CIA? DiEugenio was on to you. You can't get anything past that guy.
Geez, what can I say, I'm busted!
I was having a closer look at the people in the "fraction of a second" Lovelady GIF, and they all seem to have at least a slight reaction, the man next to the entrance wall seems to lean back, the lady in black's head similarly seems to lean back, the person in the front with their hand shielding their eyes seems to slightly lower their hand to cover their eyes and Lovelady of course is ducking down. Lovelady's testimony says that he heard the "fireworks" and didn't realize that there was any weapon till later.
Mark Tyler made a GIF showing the P.Parade and when the pictures/films were taken, the graphic's timeline is based on a constant speed of the vehicles and compared to what can can be seen in Zapruder and the films/photographs, so there will be some expected discrepancies of a second or two but it looks to be pretty close. Anyway Mark has the start of Wiegman about half a second before the head shot and allowing for the above expected error tolerances, the above 4 reactions may have been to the sound of the head shot that resulted in Z313?
JohnM