There a a number of two shot witnesses or witnesses who place the second shot as the head shot, notably SA Kellerman, James Jarman, SA Hickey and SA Kinney. Gayle Newman, in the 50th anniversary interview in the 6th floor museum, stated, between the 12 to 14 minute mark, she never actually heard a third shot.
Marilyn Willis (FBI Report on 6-19-64) "...when the motorcade passed on Elm Street in front of where she was standing she heard a noise that sounded like a firecracker or a backfire. A few seconds later she stated she heard another report and saw the top of President Kennedy's head "blow off and ringed by a red halo." She stated she believes she heard another shot following this."
Regarding witnesses who saw the President react to the first shot. I was looking at Pat Speers website where he has a quite startling list of witnesses who reported Kennedy reacting to the first shot in such a way it indicated he had been hit when compared with his known reaction from the Zapruder footage. He concludes:
"When one performs even a cursory review of the statements regarding the movements within the limousine at the time of the first shot,
one finds that 44 of these indicated Kennedy had a reaction to the first shot."
http://www.patspeer.com/chapter5%3AthejigsawpuzzleHowever (and I hope I'm not getting this wrong), he seems to be of the opinion that this first shot occurred at z190. At the moment I've not read enough to understand why he's come to this conclusion (if that is indeed his conclusion) but I believe the Z-Film refutes this conclusion. In the first post of this thread there is a partial Z-Film focussing on the SS men in the Presidential follow-up car. It stays with them until z207, almost one full second after z190. At least three agents describe reacting immediately to the first shot (a reaction we can see in Altgens 6) but in the Z-Film posted there is no such reaction. I find it highly unlikely there would be no reaction for almost a full second from any of these trained agents from a shot at z190.
One of the main arguments for a shot before JFK goes behind the Stemmons sign is a quick 'head-snap' from right to left, presumably a reaction to the sound of a shot. However, on closer examination I believe it can be shown no such head-snap occurs.
Look at the hairline of JFK in the following frames:
In the top pic (z207, just before he passes behind Stemmons sign) the parting in his hair on the left side of his head can just be made out. Certainly the way his fringe sweeps up to the parting is clearly visible.
In the second pic (z225, first full frame of JFK emerging from behind Stemmons) his parting is not so visible but the sweep of his hair up to it is.
In the bottom pic (z230, JFK facing straight ahead) the part of his forehead revealed by the sweep of his hairline up to the parting is no longer visible.
Far from turning to his left JFK is still looking to his right as he passes behind the Stemmons sign (z207).
There is no head-snap to the left and, therefore, no reason to suspect JFK is reacting to anything.