Mystery lines across top and bottom of neg and print

[Ben Calwell]

I shot a still life on Tri-X using my Linhof Tech III 5x7, and the resulting image has faint, shaded lines (looks like a group of 3) across the top and bottom (see image). On a different vertical shot, the same lines are along the sides. Could internal bellows flare have caused this? I developed the sheet by itself in a tray, so I’m ruling out any kind of serge pattern or “bromide drag.” I used the modeling lights of my strobe system to light the scene — a soft box to left, and a gridded light on the background. It was a 25 second exposure at F22.
The film was outdated by several years and was in a hot attic environment, which might factor into it. The bellows is light-tight. I used a 210mm lens, and shaded it from the left.
It almost looks like the cast shadow pattern from blinds, but that’s not the case.
Any ideas?