thanks Paul I will try cleaning the thin rollers as well. Looking at the back it seems like they are the only possible source of the scratch - nothing else really comes in contact with the emulsion.

The scratch is consistent across many many rolls with this back, and never showed up on another back I was using, so I am confident it is the back and not user error.

The flare thing shall remain a mystery - if you're curious here is the original thread with pictures: