Depending on the exact purpose, the marks could have been erased from the plate/stone. Since the stone is limestone, and the parts that are intended to print are on the stone with an oily substance, erasing is pretty easy. The press operator just "erases" it with a limestone stick.
Most of these marks were probably intended to be left on. It's likely they were intended as marks showing where the edges of the sheet should be trimmed off. Of course, in practice the sheets were cut face up, so the marks were useless.
Ones like on the Elberfeld above may have been intended to be erased. They're more like layout marks than indicators where to cut.
The ones that were commonly erased were the layout marks that were at the center of the design edges on the front that were there to make sure each color was laid out properly aligned.
Steve B
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