John, I'm guessing Z145 maybe?

You could be right given the view of JBC's left hand would be blocked if it is raised at the frame. His head is turned to his right as it is in the photo.
I wasn't fishing for confirmation of by belief that the first shot was fired at Z147 but if that slide was taken at Z145 and if Harold Norman was correct that the first shot was fired just as JFK was brushing his hair, it does fit. There's two "ifs" in that analysis so I'm not claiming it as proof of my belief, but it does provide some support. Given Norman's close proximity to Oswald, there would be almost no lag time between the shot being fired and Norman hearing that shot.