After looking at the above briefing board, I noticed what at first seemed to be a sprocket hole on the lower left corner of Z314, which could add evidence that the Zapruder film that they had on the 23rd was the unmasked original. So I had a look online for the SS copy and the best I could find was from a video posted by "The JFK Theorist" but frustratingly he wasn't sure where he got it from.
"NOTICE: video 87.38 is NOT sourced from the National Archives, but I cannot remember where it originates from. I added it as a replacement because 87.38 is not available online and probably will not be for a while. I cannot and will not confirm that the replacement is the same as 87.38, but I believe it is extremely similar and originates from the Secret Service, presumably the Dallas Secret Service." The JFK Theorist-YouTube.
Anyway, I used this copy and overlayed the Z314 from the briefing board to see if the sprocket hole lined up with the respective Z314 and the holes don't line up.

But what is interesting is that instead of cropping the original at the limit of the sprocket holes, the briefing board Z314 was cropped on the LHS at the exact limit of the masked off area, on what is most probably a copy of the SS original. Also worth noting is that after reviewing several images of 8mm film that has the sprocket holes masked off by the camera or by post processing, this left edge isn't always consistent in relation to the sprocket holes. And additionally, the briefing board photo wasn't exactly square, so I applied corrective software to square it up with Z314 and TBH it's still fractionally off but insignificant enough as to not affect the above conclusion.

JohnM