I think it's a light leak and coming through the rangefinder window closest to the shutter release. That leak accesses the film only when the shutter is open but it comes from above, and is thus able to flow outside of the frame. The cause is likely a missing seal in the top of the camera.
Look at the lighting in the pictures that show this defect and imagine how that light falls on the front of the camera during exposure. It's the light that strikes the camera front from above the horizon and it's being reflected down into the body to the lens side of the shutter.
I bet if you focus, place your finger over that rangefinder window, and then release the shutter you won't see this leak on your film.
Then again, I could be wrong.