As the leak is only ever within the frame, never between frames, and never above or below, I believe it is reasonable to rule out light penetrating the back, sides, top, or bottom of the body. You could test for leaks from the front by covering the body in a changing bag and making two exposures. First by protruding the lens from the bag and exposing the lens to bright light for several minutes after cocking the shutter and before releasing the shutter while the camera is completely inside the changing bag. Advance the shutter. Secondly by releasing the shutter with the camera inside the bag and then before advancing the film/cocking the shutter, protrude the lens from the bag again and pointing the lens at the bright light again for several minutes.
This test will tell you if the leak is from the front, and if it is, which curtain is the problem.