A few years ago, I was looking into the Zapruder film speed and found a few FBI documents on the MF site relating to this - it transpired that, IIRC, Zapruder had initially said that his camera was set to run at 48fps when filming the JFK motorcade!! The FBI then had him sign a statement to the effect that he was "in error" when he said this and that it was set to run at 16fps as per their investigations (did he also say this in a tv interview? I seem to have a vague memory from somewhere!?!). When I looked further into this, I found the same scenario for Nix - he said his camera was set to run at 24fps but the FBI said it must have also been set to run at 16fps, same as Zapruder's. I corresponded with Gary Mack on the subject via e-mails but he seemed to "agree" with the FBI's version!
Again, please bear in mind that this is from memory from a few years ago but it is the case though the figures may be slightly out (but I don't think they are).