Normal licence plates in Montana have a one or two digit number to start the plate number. The number represents the county in which the car is registered. The county’s are roughly listed by population.
For example, Missoula is county 4, Flathead is 7, Lake is 15, Ravalli is 13 and so on.
So the next time you see a lambo with 31-XXXX plates ask them how life going in Anaconda.