A friend and fellow collector put together an incredible website with information about Washington State license plates. In addition to county information you will also find great history on prestates, temporary license plates and more. Excerpt below and image provided from dannyslicenseplates.com. His website is worth a visit!
In 1935, Washington began to issue license plates by county. A one- or two-letter prefix denoted the county, and letters were assigned by the order of population, with King County, the most populous, receiving letter A, followed by Pierce with B, and so on. After letter Z, the 13 smallest counties received two-letter prefixes derived from the county name.
For the first six years, the passenger vehicle class was the only license plate type to receive a county code. Starting in 1941, truck and trailer formats were adapted to include county codes. In 1954 the system remained the same, except the county code became a suffix.
With the adoption of a modern ABC 123 numbering format for passenger vehicles in 1958, the county codes were modified to a three-letter format as shown below (county coding was discontinued for truck and trailer plates). By 1980 the system became too complex to manage and the state discontinued the practice.
