A match game between the Liberty and Defiance Base Ball Clubs was played on Sunday, June 18th. The match took place on the Defiance Club's grounds, at old Camp Jackson.
-St. Louis Daily Press, June 21, 1865
This is the earliest reference that I've seen to baseball being played in the Camp Jackson area. By 1867, the Veto Club would be playing their regularly and in 1874, Thomas McNeary would build the Compton Avenue Grounds on the site.
One interesting thing of note is that pitching for the Liberty Club that day was "J. Dillon." It's impossible to know for certain if this was the illusive John Dillon of Red Stocking fame. Dillon would have been about fifteen years old at the time which is probably old enough for him to have been a member of the club. As much as I'd love this to be Packy Dillon's brother, it's impossible to say for sure. And I can't begin to tell you how frustrating that is.