Well, lemme see...running down the list of states I've recently lived in and registered vehicles...
CA, AZ, CO, NH, HI, no county designation.
FL, NM, GA, bottom center.
ID--number/letter designation denoting alphabetical order of the county in that letter: i.e., Bear Lake County would be 2B (Bannock is 1B).
WY--Number designation for the county. Apparently the number designation came about based on property valuation ranking in the state as of 1929. Whateva.