Try opening the back in a dimly lit room and pointing a bright flashlight toward the lens from the front. Look at the curtains from the film side. It's possible that the shutter curtains are leaking light, not from pinholes (which would be more defined) but through either cracks in the lining or from being threadbare. If you are getting these leaks over a period of time and the image is still this definable, it could be more of a fogging efect than a bright flash of light that is getting past the curtains.
If the effect is limited to the negative frame and not getting to the sprocket holes, the leak is coming from in front of the shutter.
-Paul