Alabama began using numeric license plate prefixes in 1955. If you know the numbering system, you can identify the county that issued most plates.
Codes 1 through 3 correspond to the three most populous counties in 1955: Jefferson County (county seat Birmingham), Mobile County (county seat Mobile), and Montgomery County (county seat Montgomery—the state capital). The remaining 64 codes are assigned to counties alphabetically.
The motto "Heart of Dixie" has appeared on most Alabama license plates since 1955, and a heart has appeared somewhere on the plate.
Of special note is St. Clair County, which has two fully functional county seats!