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The U.S. state of Maine first required its residents to register their motor vehicles and display license plates in 1905.
Passenger plates 1905 to present
In 1956, the U.S. states and Canadian provinces came to an agreement with the Automobile Manufacturers Association that fixed the size for license plates for vehicles, except those for motorcycles, at six inches in height by twelve inches in width, with standardized mounting holes. The 1955 (dated 1956) issue was the first Maine license plate that complied with these standards.
References
Vehicle registration plates of Maine Wikipedia(Text) CC BY-SA