I saw a re-tweet for this “Man From Mars Radio Hat” advertisement and—while it’s awesome as a bit of nostalgia on it’s own—it also reveals a bit about our (US) Postal history:
The “Zone” field is not for modern ZIP codes, but for postal zones which were implemented back in 1943. The Smithsonian [online postal museum](http://postalmuseum.si.edu/) explains what they are in their [excerpt](http://postalmuseum.si.edu/zipcodecampaign/p2.html) from Carl H. Scheele’s book, _”A Short History of the Mail Service”_:
>_The basic idea of postal zoning came into being in 1943, in the midst of World War II. At this time, U.S. annual mail volume was growing steadily (the years between 1940 and 1943 saw an increase from about 28 billion pieces of mail to about 33 billion pieces of mail). In addition, post offices across the country were forced to hire inexperienced clerks as more experienced staff members went off to serve in the war. The creation of a postal zone system was deemed necessary in order to improve the accuracy and efficiency of mail sorting and delivery. Zone numbers were implemented in 124 of the country’s largest urban areas, and individual delivery districts within these areas were given one or two digit codes, to be written as part of the address after the city name. These early postal zone codes faced little opposition and caught on quickly with businesses and the American public._
Simpler times and smaller world back then.