plate-mark
/ˈpleɪtmɑːk/noun
1= hall-mark. Now rare.
- Between 1784 and 1890, plate-marks also included the head of the reigning sovereign as a mark showing that duty had been paid..
2An impression left on the margin of an engraving by the pressure of the plate, and regarded as a sign of the quality and authenticity of the print.
Origin
Mid 19th century; earliest use found in Peter Simmonds (1814–1897), newsagent and journalist. From plate + mark.