Indications point very strongly to Shaw, of the Detroit League Club, as the new pitcher engaged by the St. Louis Unions, and whose appearance in the box is promised on July 29. On last Tuesday he was fined $30 by Manager Chapman for not reporting for practice that morning, and on Wednesday evening he purchased a ticket for Boston, boarded an East-bound train, and left Detroit. In answer to a notification that if he did not return forthwith he would be expelled, he rather ironically informed the officers of the Detroit Club that if they would withdraw the fine that was imposed and advance him $300 he would return. He knew very well that those conditions would not be acceptable, and probably dictated terms because he felt that he was in an independent position and could do so with safety. If he comes to St. Louis it will be the first retaliatory blow that the Union Association will have dealt the national associations for their contract breaking and it will be a telling one.-St. Louis Globe-Democrat, July 14, 1884
St. Louis did not sign Dupee Shaw but he did sign with the Boston Unions so I guess this was UA's first retaliatory blow against the NL and AA. I'm not exactly certain that it was a telling blow one but who am I to judge?
Also, I should add that Dupee Shaw is a great baseball name and he was not a bad pitcher. He started over 200 games in the major leagues and finished his career with an ERA+ of an even 100. He wasn't a great pitcher by any stretch of the imagination but he looks like a legitimate major league player to me.
Pulling Major League Baseball Profiles off the shelf, Peter Morris and David Nemec wrote that Shaw's "numbers suggest a pitcher unable to retire ML batters on a regular basis, but nothing could be farther from the truth. Instead, Shaw's career resembled those of many pitchers whose careers overlapped with the advent of legal overhand pitching: He could be all but unhittable when paired with a catcher who could handle his offerings but at the batter's mercy when he lacked one." They quoted Jack Gleason as stating that Shaw was the swiftest and trickiest pitcher he had ever faced.