Way back in 1940, Oklahoma decided to specify the county of registration on the state’s license plate as part of the actual number, and the counties would be listed in order of population: Oklahoma County was 1, Tulsa County 2, and so on down to 77. It perhaps did not occur to them that these numbers might be variable, and in 1963, after a couple of Censuses had realigned them, the state switched to a two-letter county code followed by up to four digits. This would not be enough plates for the two biggest counties, and so Oklahoma County was assigned all combinations starting with X (later also Y), and Tulsa those with Z. But this wasn’t enough either, since several counties ran out anyway, including Cleveland and Tulsa, and after 1985 they gave up on the whole idea.
Until 2010, anyway. Per Senate Bill 318, passed last year, all new stickers issued henceforth, those expiring in 2011 and thereafter, will once again bear a two-letter county code. Presumably there will be different combinations for Coal, Comanche and Cotton.
This does not mean, however, that tag numbers can be duplicated from county to county, as Kansas had been doing with vanity plates.