We moved to Dallas Texas in 1963. At that time our address was followed by Dallas 30, Texas. Some time in the late 1960's we started using the full zip code which was 75230. I think it was around 1968. (Just checked Wikipedia it says 1967)
I had forgotten about that "coding". It preceded the ZIP code system, and was mostly for large cities. I don't know what they called it or how it was determined.
Jim