The shape of it looks like something that could have happened on the press. Sometimes they have to use a bit of solvent on the rollers that spread out the ink so it goes onto the plate evenly. Under the right conditions it can dry out and make the print get a bit spotty. So the rollers get wiped with solvent. It's a bit unsafe to do while the press is running, but it's done.
If the solvent drips onto the offset blanket you get a mark like that. Ink spread out where the drop is, and a spread area after that.
Similar marks come from water dripping on the plate. The water is in a tray that gets filled and drops n the plate can lead to an area where the ink has "floated" off the plate, but hasn't been entirely rejected.
Interesting mistake, I have a few on more modern cards.
Steve B
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