Identifying diplomatic license plates started as a novelty, but my curiousity grew when I found some country codes were not documented.
Of course, I had to start my own catalog of diplomatic placards.
Esoteric U.S. Diplomatic License Plate Codes
This is perhaps the most thorough (although horribly formatted) information on U.S. diplomatic license plates: U.S. Department of State Office of Foreign Missions Diplomatic License Plates (http://dcplates.com/OFM.htm).
Here is one of top sites containing a list of license plate prefix codes and the corresponding country or government body: Codes Found on U.S. Issue Diplomatic and Consular Plates (http://www.pl8s.com/diplo.htm). This is a good start, but I have encountered several diplomatic plates in the wild which are not on this list.
There is some information available in the U.S. Department of State’s website: Diplomatic Motor Vehicles Circular Notes and Notices (https://www.state.gov/circular-notes-and-notices-diplomatic-motor-vehicles/).