A Z124 shot would mean JBC did not start his turn to the right until 40 frames later at Z164. That's over two seconds. A 147 shot would have JBC reacting in just under one second. That seems more plausible to me.
Connally made a quick head turn to his left at Z-150 before turning his head back to his right.
That's why he's "facing straight ahead" in the famous Croft photo which was taken at Z-160, iirc.
(FWIW, a horizontal head turn isn't indicative of a "startle reaction," but a conscious reaction after a startle reaction.)