Zip Code

In 1963 the US Post Office introduced the ZIP Code.
The government was busy educating the public on how to use it and what it was for. They even introduced Mr. Zip, whose lengthy stride eventually led to multiple knee surgeries, but much quicker mail delivery. This ad appeared in the October 1963 issue of the Chamber of Commerce's Greater Sioux Falls. I think the text explains things pretty well. This was the start of streamlining the postal service to make it more efficient in the United States.
The use of the ZIP Code made it necessary to standardize the two-letter abbreviation of states. There just wouldn't be enough room on the last line of the address to put the name of a state and the ZIP Code. Before the ZIP, South Dakota might have been shortened to S. Dak. or So. Dak., but not likely South Dakizzle, as the kids today now say (I'm guessing).
Greater Sioux Falls, Oct. 1963, Courtesy Mark Helberg