Just throwing this into the mix,
ATA's paint scheme on their shiny new B737-800's is somewhat similar to MOL's. The ATA aircraft are equipped with spiffy looking winglets.
ATA is hubbed out of Midway (CatI). Maybe it was one of these 737's you saw.
ORD is an ideal alternate for ORD (Cat III).
Ryan flew a postal contract using B727's. (I believe FedEx now "owns" that flying). They usually go to the freight terminus on the southwest side of ORD.
Any chance the call sign referenced such a B727 whilst you thought it applied to a B737 and put two and two together to come up with five?
Just wondering.