I too started frequent business travel from STL, beginning in 1990.
Remember flying into C concourse on a Friday p.m.
Place so crowded you could hardly walk in a strait line.
Two Ambassadors Clubs @ STL.
Moved to CVG (i.e. DL) in Jan 1994, just after getting my red card from TW.
I was in STL a few months ago with a lot of time to spare.
Walked from the East Terminal to the Main.
Saw a plaque on the wall half-way through the darkened D concourse.
It was the dedication plaque from St. Louis to the employees of Ozark Airlines commemorating the opening of the D concourse.