Help - has anyone seen (and cured) a line like this before?

[Stub Mandrel]

I have an odd artefact I think may be an internal reflection:

Here's my stack from DSS, just balanced and stretched, with control frames, and converted to jpeg:

And here's just the light frames stacked and stretched to exaggerate the line:

It isn't light leak from the viewfinder (covered) or stray light ( a foot long flocked dewshield saw to that).

I'm sure someone else had a similar issue. Was it a reflection, did they solve it and if so, how?

[Alien 13]

Is the artifact present on flat frames with the same optics? I would also try a quick sky shot with no lens etc attached to the camera and examine that too.

Alan