Help please! Mystery light leak - causes?

[Gish]

Hello everyone, hope you can help. Have been experiencing some odd light leaks - or flare. I recently purchased a great condition Wista and a couple of lenses - also mint looking - yet have had results come back with a very odd flare I can't figure out. I have inspected the bellows quite thoroughly in a dark room with a flashlight - front to back, racked out, raised and twisted and can't see a problem there. I haven't kept notes on my shots, but I "think" it only occurs on my 90mm Nikkor lens which is on a recessed board. Shots on the Rodenstock 135 are fine. The 90mm is not scratched or even dusty when given the flashlight test. Lens appears fine - blades are not sticking and the exposures seem okay aside from the flare. The lab I use for processing is a pro lab and they have not had it on anyone else's work and are equally mystified. If it was a light leak, would the film not just be fogged? This looks focused to me, and is in a similar place each time, although might have rotated with the back (on the portrait orientation shot...). Apologies, these are contact sheets snapped on my phone.

Ideas?

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[Oslolens]

First one is excellent, plan better for the next shots, maybe sun in a better position ;)
It's bellows, camera, back, lens board, front standard or shutter.
The shutter has two positions when closed, did you test both before and after firing?

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[Gish]

Haha thanks! Well not too much sun in London this year! for these shots didn't really have the luxury of waiting for light - blocking pavements and a big camera in a rough neighbourhood not very subtle...;D
Yeah I tried looking at the leaks, can't seem to see any. not really sure how to check the shutter.

[Oslolens]

In a dark room, flash light against the lens in both shutter positions.

[Conrad . Marvin]

Sometimes you can get flare of that sort if you have a wide angle lens and a light source just outside the image area. Even with a MC lens.