In 1936, the state of Montana began using the county license plate prefix numbers still used today. Numbers were assigned to counties based on population in about 1930, from most populous (Silver Bow County—county seat: Butte) to least populous (Lincoln County—county seat: Libby).
If you know the county numbers, you can identify the county that issued most Montana plates. (Exceptions include personalized and special issue plates, for example.)
The silhouette of a bison skull separates the county prefix from the remainder of the plate number. On some plates, the county number is immediately followed by an alphabetic letter.