When did the Post Office change the state abbreviations letters?

Answer 1

In 1963, the Post Office Department implemented the five-digit ZIP Code, which was placed after the state in the last line of an address. To provide room for the ZIP Code, the Department issued two-letter abbreviations for all states and territories. Publication 59, Abbreviations for Use with ZIP Code, issued by the Department in October 1963, told why. Two letter abbreviations were provided by the Post Office Department as an aid to mailers in accommodating ZIP Codes with the usual City-State line of addresses. The abbreviations are based on a maximum 23-position line, because this has been found to be the most universally acceptable line capacity basis for major addressing systems. A breakdown of the City-State-ZIP Code line positions is as follows: 13 positions for city, 1 space between city and State designations, 2 positions for State designation, 2 spaces between State designation and ZIP Code, and 5 positions for ZIP Code. Only one change has been made to the abbreviations issued in 1963. the two-letter abbreviation for Nebraska, originally NB, was changed to NE to avoid confusion with New Brunswick in Canada.