Remove the back and lens, point it at an evenly lit area like a wall and trip the shutter throughout the speed range. On most FP shutters a variance can be seen.

I agree with mwdake in thinking the shutter runs vertically. Since the area's at the side of the frame and soft edged I'd think more of an obstruction, either in the body of something hanging in front of the lens. Strap, sleeve or similar.