You can put the negative into the back as it was shot and the location of the leak will become obvious. Along with the dark slide seal, the Horseman backs have a seal at the hinge and at the latch. Given that the leak appears to be offset from the dark slide gate opening and runs the whole width of the film, I would check the latch and hinge foam.
There's a small leak at the bottom '3' and a bit to the right of the bottom 4, also. Check location for that. Maybe related to teh double stroke film advance, a small pause?