JFK visibly reacted to Oswald's first, missing-everything, shot by very quickly turning his head towards Jackie in Z-143 and then quickly turning it back to his right to look at and wave to the spectators.

Of course, he reacted a lot more noticeably when he leaned forward and to his left and "clutched at his throat" after he and then JBC were struck by CE-399 around Z-222.

You can read whatever you want into JFK's head turn. There are any number of plausible reasons he turned his head. There is no definitive proof as to why he turned his head. What I do know is after the first shot, he resumed waving to the spectators on the north side of Elm St. as if nothing was wrong. I seriously doubt that would have been his reaction if he had recognized the sound as a gunshot as JBC did. Although it seems puzzling, it is clear that some people reacted to the first shot and some didn't. Brehm can be seen clapping in the background in the background after JFK had been hit by the second shot. We can only guess as to why some people are seen reacting and some are not. My guess is the roar of the accelerating motorcycle escort muffled the sound of the first shot. The people who showed the least amount of reaction to the first shot are those closest to the motorcycles. That would be Jack, Jackie, and the agents in the Queen Mary. That probably influenced some of the spectators along Elm St. as well.

Getting back to the theme of the thread I would like to present a partial analysis of the Altgens 6 picture to outline what this reveals about the first shot. The main thrust of the analysis centres around this close-up of Altgens 6:



The above pic is a close-up of the Vice Presidential car. From right to left the occupants of the car are;

Senator Yarborough (sat on back seat): He appears to be smiling but it's difficult to say for sure as most of his face is obscured by part of the car.

Hurchel Jacks (driver): Definitely not smiling, he seems focussed on the road ahead.

Ladybird Johnson (sat in centre of back seat): Smiling away without a care in the world, seems to be looking into the crowd.

Lyndon B Johnson (sat to Ladybird's right): He is difficult to see, there is just the side of his head and left ear visible, he appears to be leaning forward, possibly interacting with Rufus Youngblood.

Rufus Youngblood (front passenger seat): Also difficult to see as his head is in shadow and partly obscured by the motorcycle in front. The profile of his head is smooth and round suggesting he is turned to his right and we are seeing the back of his head. This also suggests he may be interacting with Johnson who appears to be leaning forward from behind.

The main point of this initial analysis is to establish that SA Rufus Youngblood is sat in the front passenger seat at the time of the Altgens 6 picture:




The grey curved shape on the left is part of a motorcycle windshield. The black rounded shape behind this is the side profile of Youngblood mainly showing the back of his head. Just behind Youngblood's silhouette is what little we can see of LBJ. The most distinguishing feature is his left ear, the bottom of which is partly covered by shadow. It appears LBJ is leaning forward from behind and Youngblood is turned to his extreme right as if they are interacting in some way.
There can be no doubt that Altgens 6 shows Youngblood still sat in the front seat at z255.

Now onto events that occurred in the immediate aftermath of the first shot, in particular the report that SA Youngblood jumped on top of LBJ to shield him. This brings us to various eye-witness accounts, not a particularly favourite area of mine as accounts can often be contradictory and this is no different.
In an interview with William Manchester, Senator Ralph Yarborough is adamant Youngblood did not jump into the back seat to cover LBJ:

(4-6-64, 11-11-64, and 5-26-65 interviews with William Manchester, as represented in The Death of a President, 1967)
(On whether or not Youngblood climbed into the back seat) "Ralph Yarborough goes further: he insists that Youngblood never left the front seat. It is the Senator's recollection that the agent merely leaned over the seat and talked to Johnson in an undertone. He contends that there was insufficient space in the rear for Youngblood." [patspeer.com]

And Yarborough should know as he was sat in the back seat with LBJ and would surely have noticed! However, as we shall see, there is copious eye-witness testimony that Youngblood did indeed jump over into the back seat to shield LBJ:

Hurchel Jacks [driver of VP car]

"At that time I heard a shot ring out which appeared to come from the right rear of the Vice President's car. Mr. Rufus Youngblood, the Secret Service Agent riding in my car asked me what that was and at the same time he advised the Vice President and Mrs. Johnson to get down. He climbed to the rear of the seat with the Vice President and appeared to be shielding the Vice President with his own body. At that time I heard two more shots ring out. At that time he told me to get out of there as fast as possible."

Clifton Carter [VP aide, front-middle VP follow-up car]

 At approximately 12:30 p.m., our car had just made the lefthand turn off Houston onto Elm Street and was right alongside of the Texas School Book Depository Building when I heard a noise which sounded like a firecracker. Special Agent Youngblood, who was seated on the right-hand side of the front seat of Vice President Johnson's car immediately turned and pushed Vice President Johnson down and in the same motion vaulted over the seat and covered the Vice President with his body. At that instant Mrs. Johnson and Senator Yarborough, who were riding in the back seat along with the Vice President, bent forward. Special Agent Youngblood's action came immediately after the first shot and before the succeeding shots.

Jerry Kivett [SA, right-front VP follow-up car]

Once we left the area, I could see all three cars--the President's car (I could not see any principal party and could only see Clint Hill on the back of the car. The follow-up car, with some agent holding the AR-15 pointed in the air--The Vice President's car (I could not see the Vice President, but could see ASAIC Youngblood lying over the area where he had been sitting--I don't recall seeing Mrs. Johnson or Senator Yarborough)

Warren “Woody” Taylor [SA, left-rear VP follow-up car]

I recall hearing SA Kivett telling the driver to "go, go, stay right behind the car." During all of the aforementioned, I could see ASAIC Youngblood, in the Vice President's car immediately in front of us, jump to the back seat and cover the Vice President.

Lyndon B Johnson [VP, rear-right VP car]

After we had proceeded a short way down Elm Street, I heard a sharp report. The crowd at this point had become somewhat spotty.
The Vice-Presidential car was then about three car lengths behind President Kennedy's car, with the Presidential followup car intervening.
I was startled by the sharp report or explosion. but I had no time to speculate as to its origin because Agent Youngblood turned in a flash, immediately after the first explosion, hitting me on the shoulder, and shouted to all of us in the back seat to get down. I was pushed down by Agent Youngblood. Almost in the same moment in which he hit or pushed me, he vaulted over the back seat and sat on me. I was bent over under the weight of Agent Youngblood's body, toward Mrs. Johnson and Senator Yarborough.

Ladybird Johnson [wife of VP, centre-rear VP car]

...suddenly there was a sharp loud report--a shot. It seemed to me to come from the right, above my shoulder, from a building. Then a moment and then two more shots in rapid succession. There had been such a gala air that I thought it must be firecrackers or some sort of celebration. Then, in the lead car, the Secret Service men were suddenly down. I heard over the radio system, "Let's get out of here," and our Secret Service man who was with us, Ruf Youngblood, I believe it was, vaulted over the front seat on top of Lyndon, threw him to the floor, and said, "Get down."
Senator Yarborough and I ducked our heads.

In light of the amount of eye-witness testimony that Youngblood jumped into the back seat and covered LBJ I find I must treat Yarborough's observation as some kind of aberration. If we accept the weight of evidence of eye witness testimony on this issue it can be concluded, at this point, that Youngblood leapt into the back to cover LBJ and that this moment had yet to happen in Altgens 6 as Youngblood is still sat in the front passenger seat.
My analysis of the various eye-witness accounts reveals the order of events in the immediate aftermath of the first shot (there are some slight variations in accounts, Ladybird's account seems somewhat jumbled for instance):

First shot
Jacks hears Youngblood ask "What was that?"
Youngblood turns in his seat and pushes LBJ down
He shouts at everyone to "Get down "
He jumps into the back seat and shields LBJ
Ladybird and Yarborough duck down
Two more shots ring out

When describing Youngblood's actions Carter and LBJ use the word "immediately", indicating his reaction to the shot was quick. Jacks, Carter and LBJ report that Youngblood was in the back before the second two shots were fired. What does this tell us about Altgens 6?

Altgens 6 was taken at z255. JFK has been shot through the throat. Youngblood is yet to push LBJ back down and leap on top of him. When I look at Lady bird smiling without a care in the world I get the strong impression Youngblood has yet to shout at everyone to "Get down".
This is surely an indicator that the first shot has only just happened and, as many other witnesses confirm, the first shot hits Kennedy.
Altgens 6 rules out an early missed shot!

Over eighty witnesses provided evidence that is inconsistent with a shot at that point. Even Croft said his photo was not only before the first shot, but he had time to wind his camera and get ready to take another before the first shot.

"Evidence," my you-know-what.

The first shot at "Z-124" sounded different from the other two, probably because the muzzle of Oswald's short-rifle was inside the building for the steeply-downward-angled shot when he fired it.

A lot of people thought it was just another motorcycle backfire, or a firecracker, or even a tire blowout. That, and the fact that Dealy Plaza is an echo chamber, threw off a lot of people. Unfortunately, eighty of them provided their mistaken opinions.

So, your insistence that they were all independently, wrong but mutually consistent with each other because you can read Jackie's face in the Croft photo is difficult to understand.

Is English your second language?

In any event, face reading is not a science and is nothing more than speculation with zero evidentiary or persuasive value.

You said the reason Jackie turned her head far to her left between Z-145 and Z-161 is because "she was acknowledging the crowd."

If that was the case, why wasn't she smiling and/or waving in the Robert Croft photo?

Why did she look so concerned?

IF Secret Service Agent George Hickey was leaning over the side of the car [between Z-150 and Z-170], it must have been to get a better look at something unrelated to a rifle shot, possibly people who were standing in the roadway.

I guess you haven't seen how far down he was leaning in Z-160.

Maybe he was barfing, or picking up some litter, right?

https://assassinationresearch.com/zfilm/z160.jpg

Just to take one example from your litany of lame rationalizations, you say "Jackie moved [turned, to be more precise] her head [to her left starting at Z-145] to acknowledge the crowd."

Rhetorical question: Is that why she had a concerned look on her face as she was looking directly at Robert Croft in his well-known Z-161 photograph?

Seems like a strange way to "acknowledge the crowd."

Over 80 witnesses provided evidence that is inconsistent with a shot at that point. Even Croft said his photo was not only before the first shot, but he had time to wind his camera and get ready to take another before the first shot. So your insistence that they were all, independently, wrong but mutually consistent with each other because you can read her face is difficult to understand.  In any event, face reading is not a science and is nothing more than speculation with zero evidentiary or persuasive value.

By the way, you never gave a rational answer to my question:

Why did Secret Service Agent George Hickey lean so far over the side of the car, and when did he begin that motion?

Since there hadn’t been a shot, if he was leaning out it must have been to get a better look at something unrelared to a rifle shot, possibly people who were standing in the roadway.

[...]

Jefferson "Yuri Nosenko Was a True Defector" Morley's "JFK Facts" (sic).

Gag me with a KGB spoon.