If you fail to thread the film under the second roller, then there’s nothing to keep the film against the film rails at the right end of the frame. That would leave the image badly defocused at the left side of the photo—which is exactly what happened.

So far as I can see by looking at the photo of the open back shown (link in post #4) the felt bar (or whatever material it is) shown on the F7 is a light barrier to prevent any stray light from the lens from fogging the exposed film on the take-up spool.