john, there is a physical frame that makes the framing on film, which casts a shadow in the perforations and in the inter-frame, a temporary light leak cannot expose two frames at the same time...
Unless the light leak in question happens to occur during film advancement...
What are the film movement and shutter related mechanics like on an M2?
I've not played with one yet, but I am suspicious as to why there is such a clean masking along the film edges, but not the frame spacing. To me this looks like something is forcing a tiny shutter fault during film transport.
If it were a light leak from the back of the camera then wouldn't we expect to see bleeding into the sprocket area?
However, Abruzzi's suggestion of pressure/alignment issue allow a gap in the frame-mask would also be high on my list of sources. But then I would expect the bleeding to likely go further into the perforation area, so that's really looking like a weird bit of light...