Zapruder Briefing Boards placed in National Archives by CIA 1993
The “start” frame is well into the Zapruder film. There were many frames before that one that didn’t get the attention they deserved. The HSCA did realize this and came up with their conclusion that the first shot was much earlier than the “start” frame in this briefing board.
Yes, according to the notes (the ones I found or survived) they began with Z188. The notes claim/argue that the/a first shot was at Z190 so I guess starting with Z188 is the explanation.Notes/link is here: https://www.maryferrell.org/showDoc.html?docId=7154#relPageId=2