You many know that Prairie Farmer magazine was a weekly Illinois Farm magazine started back in 1841. I am not overly clear on the tie in but WLS has many articles in the magazine and is listed as "Listening with the Prairie Farmer, WLS the voice of agriculture". I just picked up a number of them from around the late twenties and early thirties. One has quite a spread instead of the usual page or so devoted to what is on the radio that week.
Somehow I thought WLS stood for Worlds Largest Store which was sears and was headquartered in Chicago at the time. Not sure of the tie in I will have to goggle it and see what I find.