Yeah, I've been thinking of contamination of some sort as well. I wash my tank thoroughly with plain water after fixing, and the tank doesn't really have any obstructed areas where microscopic grit may accumulate. Something in the tap water that I used for this film? Maybe, but 'gritty water' would leave unexplained that the scratches are all in the same direction on the film. Something in-camera then? No grit there that I can see (but then it would probably be too small to see). I'm at a loss!