Union Base Ball Club - The members of the U.B.B.C. are hereby notified to meet at the Play Grounds, Gamble's addition, on Monday next, 15th inst., at 3 o'clock, P.M. Punctual attendance requested. By order of the President.
-Missouri Republican, July 12 1861
And here's more evidence that the Union Club were active during the Civil War. I have a box score of a Union-Empire match from December of 1861, which I believe is their first meeting, so it's evident that the club was active throughout the 1861 and 1862 season. I guess I'll have to check 1863 and 1864.