That type of button, with that style of lettering, and including the word "Union," is usually the front of a rivet-button from overalls/coveralls/work-clothing, dating from the early 1900s. That time period was the heyday of Labor Union organizing here in the US, when workers would be very likely to favor work-clothes that said Union on the rivet-buttons. "Union Joe" might be a reference to Labor Union hero Joe Hill. Many similar variations have turned up for identification here in the What Is It? forum.