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?