Looks like those metal stamps we engraved back in junior high art class.
Per the abstract, that appears to be what they were used for. In the Mediterranean, the unique artifact the Phaistos Disk was inscribed with two boustrophodon spirals of characters (one on each face), and the characters were stamped into the clay before firing (the outlines of the block can be seen here and there). Both artifacts were at least a couple thousand years before Gutenberg. :’)