https://en.wikipedia.org/wiki/Large-letter_postcard

Wish You Were Here - The “Great Lakes” Edition from Field Notes Brand https://www.youtube.com/watch?v=fFemm4LjJbY

The Newberry Library in Chicago, IL, maintains a collection of the Curt Teich & Co.'s Art-Colortone postcards from 1898 onward. It's stored in tab divided boxes using an alpha-numeric system generally comprising a series of three letters followed by three numbers. The company sold over a billion of these postcards.