Light leaks in SQ backs
I have suffered through finding leaks in my SQ backs Replacing the seals is easier said than done if you are manually challenged. Try a process of elimination first,
Sometimes light leaks originate from sources other than the magazine.
Tape off the dark slide opening with black electricians tape .and see if it makes any difference. Replacing the seal on the dark slide opening is tricky .If you still get it, tape off the sides ,top and bottom of the magazine as soon as you load it and before you advance to the first frame. To eliminate any out side source cause the fog..
It took me three months before I discovered the focusing sheet on my enlarging easel was storing uv from my over head fluorescents and I was opening the mag on top of it.Simple answer once you find the cause