I have been following the OP findings. Very systematic trouble shooting.
Seems like there is a little light leak through the scratch of the shutter curtain. Would the black stripes show up in low light / night time shooting conditions?
Maybe OP can put on the lens cap and remove it immediately before shooting a frame to see if the stripes disappear?
Or is it possible to point the loaded camera at a strong light source for some time and then cover up the lens, trip the shutter and wind to next frame. Maybe do this for a couple of frames and then develop the negatives. If the lines still show up, your shutter curtains may need to be replaced.