My one example from that set is too beat up to tell, but I believe the set is a process other than lithography. Probably the etched blocks like some of the PCs or exhibits.
If it is it could be "stuff" stuck in that spot, or that area didn't etch. (Because of "stuff" on the plate or negative that prevented the process from working. )
If it's stuff stuck in the recess in the plate there's probably a few without the flaw. If it was a flawed plate they'll all be like that.
Steve B
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