The St. Louis base ball nine for 1875 will be composed as follows:-Catcher, Miller, of Easton, Pennsylvania; pitcher, Bradley, Easton; short stop, Pearce, Atlantic; first base, Dehlman, Atlantic; second base, Battin, Athletic; third base, Fleet, Atlantic; right field, Waite, Easton; center field, Pike, Hartford; left field, Cuthbert, Chicago; substitute, Sweasy. The men will rendezvous in St. Louis, January 1, 1875, and will go into winter training after the Boston fashion, at the Missouri gymnasium, in St. Charles street.
-Hartford Daily Courant, November 23, 1874
The question of how Charlie Sweasy ended up in St. Louis, playing for the Reds, is one that has bothered me for many years now. It's rather frustrating that I've never found the answer because I'm sure it's not that hard to find.
The Reds were, for the most part, made up of St. Louis players who had been with the club in 1874 and the core of the team had been together for two years. Sweasy arrived in St. Louis around April 12 and, according to the Chicago Tribune, he was "engaged to Captain and steady" the young Reds. What I don't know is when the Reds contacted Sweasy and why Sweasy agreed to play with the club.
The above article sheds some light on the question. It appears that the Browns Stockings, as they were putting their club together in the fall of 1874, had some interest in Sweasy and may have contacted him about coming to St. Louis. So it's the Brown Stockings who probably first gave Sweasy the idea of coming to St. Louis to play baseball. It's also probable that after the Brown Stockings passed on the second baseman, the Reds picked up the idea of bringing Sweasy in.
This is all rather speculative and based on scant evidence but it's the best explanation of why Sweasy was playing for the Reds in 1875 that I've yet to see.