In his testimony, Altgens was only sure of two shots, but guessed there could have been a third shot that fitted in between the shot he heard before he took the photo at Z255 and the head shot. Mason places his LN second shot (it narrowly misses Kennedy, going on to strike Connally) about Z271.
In 1985, "Ike" Altgens told Richard B. trask:
"My first instinct was ‘well, they’re shooting firecrackers up there,’
or some kind of celebration on behalf of the President. And then
I hear it again as the car comes on down."
So here Altgens claims he heard "firecrackers up there" (or "some kind of celebration", presumably referring to the sound of a shot -- though Altgens didn't know at that moment it was gunfire -- when the car was on upper Elm), then heard it again as the "car comes on down" which I believe would be when he took his picture.
Therefore there could have been two shots fired before Altgens took his photograph at Z255.