I've been trying to figure out exactly what would cause this sort of misprint. It's not a typical fisheye caused by debris on the plate or offset blanket. Those are color inside unprinted rings.
I believe it's from dents on the plate, or stone. Ink would pool in the dent or chip, but wouldn't print in the center if the defect was deep enough. So it would only come from one spot on the sheet, and probably only from part of the overall press run.
Having different backs should prove that groups of fronts were printed first and then backs were printed as needed.
Steve B
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