Jack Mertens (
https://www.baseball-reference.com/r...d=merten002joh), Pat O'Connor (
https://www.baseball-reference.com/r...d=oconno003pat) and Ben Wallenkamp, three young players from St. Louis signed by Decatur for the 1903 season. These photos were published in the St. Louis Republic on January 18, 1903 (pg. 21).
Pat O'Connor was the brother of Jack O'Connor (1887 - 1910), who managed for the St. Louis Browns in 1910. Jack O'Connor was blacklisted by organized baseball for instructing the third baseman to play deep against Nap Lajoie in a season-ending double header in 1910, allowing Lajoie to get 7 bunt hits in an attempt to pass Ty Cobb for the batting title. He successfully sued the Browns for $5000 in 1913 for terminating his contract over the incident.
Jack Mertens played his final season in professional baseball (under the name Jack Collins) in 1913 for the St. Louis Terriers of the renegade Federal League, under manager Jack O'Connor.
As far as I can tell, Ben Wallenkamp did not make the Decatur club in 1903, and he is not listed on any roster in Baseball-Reference.