Dist. 15 is the only ISP district where the radio ID reflects the car ID. For example, 15115 is "15-115" (Or when on the D15 channels, "Car 115") and 16115 is 15-115's portable. This holds true even for the utility cars. 15272 would be "utility car 272" (when the trooper's assigned car is out of service) but the officer uses their normal radio ID.
In the rest of the districts, the radio signatures follow the pattern <Zone><Radio Type><Officer ID>. Radio type is 0 = car, 1 = portable, 4 = motorcycle. 1005358 is 17-24's car radio and 1105358 is the portable. You'll hear them reference their (or a supervisor's) ID number when calling in for overtime or time off ... "Authority 4885, my ID is 5358" would be 17-3 authorizing something for 17-24.
Another pattern is used for the utility cars (and pool cars?) and "other radios" ... <Zone><District or Division><Unique ID>. 1317201 is a utility car for Dist. 17.
Some of the other division numbers I've been able to identify (I would love to be able to fill in the holes) are -
24 - TRT
37 - EPU
72 - Weigh stations
74 - Riverboat
Along those lines, the dispatch radios used to follow a pattern too - <Zone><Radio Type = 9><District><Dispatch Position><Radio ID>. The radio ID per talkgroup was more of a pre-consolidation thing as now they seem to use the same radio on multiple talkgroups.
For example, 1917101 was Dist. 17 dispatch, position 1, on 17-A.
For now at least up here in the north, 1904279 -1904387 is ISP Sterling (Dist. 1, 7, 16, 17) and 1904391 - 1904398 is ISP Chicago (Dist. 2, Chicago, Joliet). 2906291 - 2906297 is ISP Pontiac (Dist. 6, 8, 10)