These plates were only fitted to cars solely destined for the US market from c. 1970 onwards (I'm not sure of the exact date) in order to show that the car had been built to meet the Federal 1971 Model Year Safety and Emissions Regulations. Does your car have rivet holes on the door showing that a plate used to be present?
Service Bulletin A.42 (published in April 1971) relates. I have enclosed an extract covering the identification plate
