Wiktionary
union bug
n. (context printing English) A maker's mark showing that the work on which imprinted is sanctioned by members of a trade union. The mark often is a tiny elongated rhomboid containing and surrounded by, in miniscule typeface, identifying names and union's local chapter number.
Usage examples of "union bug".
And as they stood around or near the long table or sat on its edge sipping at their Styrofoam cups, the button continued to move from one to another of them, inscription and printed photos held close to the eyes, the little blue union bug on its reverse touched.