Dan, I’m sorry if I didn’t address what you were concerned about earlier. Hopefully I won’t still completely miss something, but if I do it shouldn’t turn any heads.
I agree with you that there are agents in Altgens reacting to gunshot sounds.
But I think there were body motions in the early Z film comparable, if not greater in magnitude, to the agents seen in Altgens. If Altgens could have snapped an up-the-road still picture on the presidential limo at about z160 he would have caught 5 heads turned looking around. And based on the Croft photo at that time with Connally and Jackie, one might say there could have been some facial expressions of concern at that time.
Now I could take a stab at what might be going on in Altgens, but I should reiterate that the first shot analysis method is based on a reaction time (better described as perception time) model to a surprise stimulus. It will not apply to after the first shot because a first stimulus can be a strong forewarning signal to subsequent stimuli and can radically change (lower) reaction times. So although the method would not be recommended for anything after a first shot, one might expect subsequent reaction times to become somewhat faster because of this dynamic. The early reactions highlighted in the article method match up exceedingly well to the surprise stimulus perception time distribution expected from the general population, that’s how it pegs the first shot triggered at half a second before z133.
But, just for the sake of it, if we play this out and assumed the second shot was triggered near z219, the muzzle blast would arrive at the limos at about z222.4. If at that point we allow 0.6 seconds for SS follow up limo agents to start voluntary reactions thereafter (this might be a ballpark number to plug in for a mid-level awareness stimulus reaction), we might expect reactions showing in the SS car starting around z233 if reacting primarily after the second shot. It would not be surprising to not see any reactions fully developed at z236. Also It would seem that this would allow enough time for some agents, if they chose to, to turn and be in the position captured at Altgens z255 (about 1.8 seconds after hearing the second shot ) if they didn’t already start looking around a little sooner than that.
At z255 I’m not sure if many of the SS agents had fully realized that that JFK and Connally were seriously shot (but Clint Hill appeared to be looking towards them and you mentioned him not reacting, and I just don’t know how to explain him to you, maybe one who is not as inclined to react?). In any case, hearing a second report about 1.8 seconds earlier than z255 and quickly after that realizing it as a gunshot might have “gotten and held” many of those agents attention at that time, looking for the source, and as seen in Altgens. Alternately though, I would guess the attention of Jackie and Nellie was on their husbands, since from z235 to z255 it looked like there was a lot of chaos going on in the presidential limo back seats with two men having just been shot and their wives reacting to their husbands. Perhaps to be expected Kellerman and Greer looked to react a little slower than the wives did by starting their turns backwards around z252ish, but that is also pretty darn close to the time of the Altgens photo.
Net, this quick run through related to Altgens is just that, a quick take. But I don’t see anything that is really inconsistent with explaining the Zapruder frame around the time of the Altgens photo, and Altgens being primarily related to the second shot rather than the first shot that happened earlier up the road.