From the 1947 Centralia Cubs Program

The Illinois State League

The Illinois State League, Class "D", was organized in early 1947 in answer to the consistently-felt need for a minor league baseball circuit in downstate Illinois.

Headed by Howard Millard of Decatur, the league is composed of six teams, including Belleville, Centralia, Marion, Mattoon, Mt. Vernon, and West Frankfort. These cities hurriedly raised money and built parks to get the league in operation in mid-May. Marion joined the league just two weeks before the opening of the season, to replace Murphysboro.

All teams have working agreements with Major League clubs: Belleville–St. Louis Browns; Centralia–Chicago Cubs; Marion–Cleveland Indians; Mattoon–Chicago White Sox; Mt. Vernon–Boston Braves; and West Frankfort–St. Louis Cardinals.

H.V. Millard was a newspaperman. Bet he wrote this blurb.

The standard source for affiliation information–Lloyd Johnson and Miles Wolff’s Minor League Encyclopedia–lists Marion and Mattoon as unaffiliated; previous editions also showed Mt. Vernon as an indy club. The 1948 Sporting News Baseball Guide listed no affiliations (for any minor league team, near as I can see); the 1947 Blue Book barely has any information about the ISL teams (and was compiled before Murphysboro bailed out). A quick check of player careers supports the affiliation claims.

Going to need to make some adjustments to MWLguide.com.