Scott, I have a videocassette called "Kennedy Assassination Film Chronology" which might be what you're looking for. I got it through the mail some time between 1985 and 1990, I would guess. Groden actually narrates part of the thing, a discussion of the movement between the concrete wall on the knoll, in extreme closeup of Nix's film. This pertains to Black Dog Man/Sitzman's couple. I just now looked at it for the first time in....decades? It starts with DCA motorcade stuff, then the various films starting at Main and Houston (like Nix) are cut together to form a pretty bad continuity. The Nix stuff is there, but incomplete. Paschall film, notably, is on the tape. I quit watching at that point, but it really trails off into a lot of stuff I didn't have time to watch the first time I put it in a VCR, let alone now. Email me if you're interested.
BTW, the black man standing with Emmett Hudson catches the light through the trees a couple of times, as he runs up the stairs, if you look closely at a really clear version of the Nix headshot sequence. Happens same exact moment as the movement just above the wall.
Happy researches,
Sim