Any idea what this mark is?

[steve B]

Is the mark black or under the black? If it's under the black or brownish it's probably a defect in the cardboard before printing, most likely caused by water.

If it's black - Then it's could be caused by a drop of water getting on the printing plate after it was wiped. The press wets the plate, wipes off excess water, then inks it, The ink won't stick to the wet places. If a drop of water gets on the plate between wiping and inking it can make a mark similar to that one. (Or worse, one of the many cards I need to scan is a card where there was a lot of excess water on the plate)

It could have happened on the red plate, in which case the mark would be red. But since its under the red ...

Steve B