Can someone explain to me how these would be caused by a problem in a horizontally travelling shutter? I'm having hard time imagining it...
This is a photo from a member of Leica Shooters Finland who had a light leak problem under the rangefinder unit (bad seal) of his M6: http://dl.dropbox.com/u/1009781/Leic...vuoto_%201.jpg
... looks somewhat similar to the first photo you posted, although the problem is at both halves, top and bottom, of the image.
Edit: this was only apparent on frames that had been "sitting there" at the film port for a while, i.e. camera on the shelf unused. Does this ring a bell in your case?