Those are really interesting. Especially the last two.
Usually a ring shaped defect is a fisheye, but it's usually a dark center with a light ring around it. The last two, especially the larger scan are like that, but the others are either the solid spot or the dark ring.
I suspect what's there is a drop of one of the platemaking chemicals that got there while the plate was being laid out. So the full spots are the earliest ones, the dark ring ones printed later after the original spot had a chance to wear, and the last two with light colored rings are actually an attempt at correcting the dark ring by stoning it off the plate. Basically erasing it with a limestone stick.
It could also be plate damage and/or a nailhead sticking up from the impression cylinder.
Steve B
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