I have two RB67 Pro S backs, and one of them is working fine. The other is producing a thin line on the film (across the bottom of the frame in landscape orientation, or left side in portrait). Has anyone ever seen this phenomenon before? It almost looks like a scanner artifact, but it is clearly present on the negatives themselves (and does not appear on films from the other back). Here is a crop of an image where it is noticeable:

line crop.png

Any ideas what is happening? Is something scratching the film?