[CCRsAC]

Mark IV - "Illinois will let you use a CALIFORNIA plate with an Illinois registration? Wow....that I wanna see...................."

Yep... right from the horses mouth (625 ILCS 5/3-804 and 804.01) and without any restriction on what state the historic plate (or representation of one) comes from:

(b) Any person who is the registered owner of an antique vehicle may display a historical license plate from or representing the model year of the vehicle, furnished by such person, in lieu of the current and valid Illinois antique vehicle plates issued thereto, provided that valid and current Illinois antique vehicle plates and registration card issued to such antique vehicle are simultaneously carried within such vehicle and are available for inspection.

The "year of manufacture" plate can be an actual plate or one that "represents the model year of the vehicle." Better yet, there is no actual registration of those plates to the car. The rule is that the car must have antique or expanded use antique plates registered to it and the registration card and the actual plate registered to the car (with a valid sticker) must be in the car and available for inspection. Other than that, it's pretty open. Surprising for Illinois... they usually take a much more hard-line approach to stuff like this.