1961-78 FORMATS No I,O,Q,Z used.* No T,U used in 1st position.*
1961-66 1 - 999 999, AA 1000 - WY 9999 (Capacity 4,185,999)
1964 Order: AA-RD BM, DL, PM omitted (3,600,999 P)
No R,S,U,V,W in 2nd position of LA-WY series.
It appears you have the choice of a pure 6 digit (3|space|3) or
AA-RD xxxx as noted.
See
http://allaboutlicenseplates.com/athome.asp?State=Illinois for lots more.
They don't seem have a reason for the alpha plates that I could find in a quick search although they may be special in some regard (trailers?).
Best to stay with "123 456"
When I did this for the home state of my SJ car they had county prefixes. It appears Ill. does not.
License plates are just plain interesting...