is it always the in the same place and the same shape?
just as an after thought, if it is it MIGHT be the film lifting up, maybe from the film spool being weird or check the back plate and see if the the pressure is the same on both sides
You are referring to the narrow dark band on the right edge of the image I assume? So that would be on the closing side of the shutter as I assume the F3 shutter moves from right to left when looking at the back of the camera? A sticky following curtain usually makes the neg darker, making the positive image lighter. So look for a film chip or even a small hair in the track that could bind the leading curtain as it starts moving. What are those two hair like critters on the right side of your image?