I have no way to see the condition of seals inside the back. This back being SD type, there are only two areas where foam seals actually exist. Hinge area and the dark slide area. One that I can see, the hinge one, looks perfectly good.
If it was a leak from light seal on the darkslide opening, I'd think the problem will appear on the other side of the frame, as that side is the closest to the opening.
I opened the back, loaded the film onto the insert, advanced it to the arrow. Mounted the insert into the back, firmly closed it, latched it, advanced to the frame 1, then pulled out the dark slide.
As you can see, something happened at the very beginning as margin area got some exposure but oddly stopped right outside of the image area, suddenly. Then, a major light leak with interruption took place over and between frame 1 and 2.
I looked at my back closely. I cannot figure this out.