I took a second look at your second picture of the Illinois. It is in a key wind case. Can you post the movement serial number? The Illinois model 1 was a key winder, but it was in a hunter case. Yours is open faced. The model 2 was hunter cased, and stem or key wind. AFAIK, the first 18-size Illinois in an open faced case was the fifth pinion model 3, and it was a key or stem wind transitional model. I have one.
Wearing my new, apparently unloved (very few people seem to appreciate the green) AT Golf.