(covers information from several alternate timelines)
An Iowa license plate
A license plate or license tag was a metal plate with an alphanumeric sequence on it that identified a vehicle.
A 1930 newspaper article quoted motorists without license tags near Lee's Summit using the excuse "I'm going to be drafted next month and don't want to spend $15 or $20." (TOS: "The City on the Edge of Forever")
In 2367, while showing Guinan his Dixon Hill holoprogram, Jean-Luc Picard complained about not being able to get the license plate of a 1948 Packard. (TNG: "Clues")
The license plate on the 1936 Ford truck USS Voyager found floating in space in 2371 was "6-T-593". (VOY: "The 37's")
The license plate of a Dodge Touring Sedan in the 2370 Keystone City was "H-20638". (TNG: "Emergence")
Background information[]
Usually the alphanumeric sequence is a government sanctioned code that indicated the geographical location or an officially recognized organization a vehicle belonged to, and is an identification aid for law enforcement agencies as well as tax authorities.
Period Dodge WC63 truck carrying an "NA"-plate in "Storm Front, Part II"
In the Star Trek: Enterprise season four two-part episode "Storm Front", several German vehicles appear with license plates bearing the prefix "NA". During the time of Nazi Germany, civilian vehicle license plates began with a code that indicated the region where the vehicle was registered. Additionally, vehicles registered to the Heer [Army] had the prefix "WH", and those registered to the SS had the prefix "SS".[1](X) "NA" was never used as a license plate prefix in real life. Conceivably, this fictional code could be intended to stand for "Nord Amerika", as the vehicles seen in Star Trek were stationed in a fictional, Nazi-occupied United States.