The frames will be exposed upside down which suggests the leak is towards the bottom of the dark slide slot. I had to replace the light seals on an SQ-A back recently and I recall that there was much more to it than I had thought. If the seals appear generally ok, then I would carry out a thorough examination of the area around the suspect area. Look for a part that shows damage, perhaps by the action of the dark slide. The varying intensity of the marks on the film probably reflects the length of time each fame was in the 'gate', and the light levels it received. I hope you get to the bottom of it. It's a great camera system. Alex.