On the ultra fine film, it's the ink on the backing, and it looks like it might be the black part of the backing, as where the white numbers are seems to look better.
I used multiple cameras, multiple backs, multiple exposures and 10 rolls of film from different boxes purchased on 2 different orders, and all of them have the issue. I also tried a different developer when I saw how bad the first few rolls were looking. So this has nothing to do with red windows in cameras. This film is junk pure and simple. I did 2 days of shooting with the stuff. I used it in a shoot where I should have known better than to use the cheap stuff but I had it and used it.