Since the 1800's, the region called "El Chamizal" in the northern part of Cuidad Juarez was located on the southern border of the Rio Grande river.
In 1855, a massive rainstorm altered the course of the river, and the residents of the El Chamizal region found themselves on the northern border of the Rio Grande,
in other words, they were now in Texas, USA.
When automobiles made their appearance, they mostly used Chihuahua plates, but some used Texas plates. Those with Texas plates were stopped and forced to pay additional taxes for Chihuahua, which, needless to say, caused some friction among the local residents.
In 1963, NPI plates were produced (no ones really knows by whom or from where), and most of the vehicles in El Chamizal were using the NPI plates.
There are pictured below, with the CIUDAD JUAREZ legend. (NPI means No Paga Impuestos - or in English, Don't Pay Taxes.)
These plates may have been the catalyst ... because in 1963, this problem suddenly gained the attention of the governments of both the USA and Mexico.
NPI plates were used again in 1964. Mexico and the USA agreed to ultimately return the region to Mexico, and the plates stopped after 1964.
(Except for the CHIHUAHUA/NPI 1965 plate shown below ... what is that exactly ??)
The transition of El Chamizal back to Mexico was finally completed in late 1968. A cement-lined channel had been built over the last few years to contain the Rio Grande, and on December 13th, 1968, Persident Gustavo Diaz Ordaz of Mexico and President Lyndon Johnson of the USA jointly set off a dynamite charge from the International Bridge, finally re-routing the river's flow.
As a result of the new (permanent) path of the Rio Grande, a small part of El Chamizal remained in the US, but the majority of it (and some additional territory) was returned back to Mexico.
And the story of the NPI License Plates comes to an end.
Thank you to the late great Roy Carson (ALPCA #17) for this information.