As I've been rereading Horne's work on the "master" film and copies chain of custody - which takes from Wrone and Trask - I keep reading about the impossibility of a 4th copy or 5th film yet repeatedly there is a film missing in the narrative.
Max's note spells out pretty clearly that Zapruder has the "master", Sorrels has 2 copies and "The third print is forwarded"
We also know that Zapruder shows a film on Sunday... but let's start at the beginning with Doug:
Zapruder departed Kodak’s Dallas Plant at about 9 PM, and turned over two of the three “first day copies” to the Secret S