I findit helpful to put the negatives back in the camera to find where light could be coming from. And DWTHomas is probably right- the leak is happening when the frame is not at the actual film gate. The leak happens when the film is coming off of the film spool or onto the take-up spool. The crisp line on the lower edge makes me think this.
So position the negatives in the camera as if it was being exposed and I bet the trouble area becomes clear.