In the United States, each State has their own license codes. For example, in California, I have a class C/M2 license (normal car + motorcycle of any size). For Washington State, it looks like they are called "Endorsements" (?) (Source:
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> but the land transport office in Samut Prakan demanded a notarized translation of the different classes and endorsements from the embassy even though none are applicable to me
Different provinces in Thailand have different requirements. You may have to go to the US Consulate, sign an affidavit saying that you have a license letting you drive [whatever] from Washington, and then get that translated and approved by the Ministry of Foreign Affairs 🙁