No, Illinois isn't a rectangular state, strictly speaking, but its markers for three-digit routes usually are the "wide" rectangular style. But Joe Reda found this one traveling eastward along US 150 one day in 2005 east of Bloomington. As Joe describes it, "I passed through Champaign/Urbana and, for some reason I don't remember, stopped in St. Joseph. I got turned around a bit, and it was lucky that I did, because I came upon an old square US 150 shield." And here it is!
Photo courtesy of Joe Reda