NATIONAL SCREEN SERVICE ("NSS")
The National Screen Service was created in 1920 to produce and distribute movie trailers ("Coming Attractions").
"Movie paper" which included 8" x 10" press stills, lobby cards, half-sheets, inserts, one-sheets, three-sheets, six-sheets and twenty-four sheets was still being handled by the studios at this time.
In 1939, Paramount Pictures contracted with the NSS to take over "movie paper" production for its films.
In 1940 RKO entered into a similar agreement. Over the next few years, the remainder of the eight major studios signed contracts giving exclusive rights to the National Screen Service to produce and distribute all of the movie paper for their films. Universal signed in 1944, Columbia Pictures in 1945, United Artists and Warner Brothers in 1946 and 20th Century Fox in 1947.
NSS continued to be the center for movie paper advertising until the mid-1980šs.
To keep track of all the materials it was producing, the NSS instituted a numbering system. All movie materials distributed through the National Screen Service bore the NSS number.
The NSS number is usually found on the bottom of the poster. Until the 1970's, the NSS number consisted of two numbers followed by a forward-slash (/) and one to four more numbers. (This number looks like the "edition" number printmakers put on prints but does not mean the same thing.) In an NSS number the first two numbers are the year of the movie's release and the last four numbers represent the sequential order of the movie for that year. An NSS number of 47/140, for example, indicated that the movie was released in 1947, and was the 140th movie processed by NSS for that year.
The NSS changed their numbering system during 1977. The same numeric breakdown was used, but the forward-slash (/) was eliminated. The first two digits of the number represent the year the poster is released and the last digits represent the sequential order of the release for that year.
To indicate a poster is a reissue for a film re-release, NSS numbers for re-released films contain the letter "R" preceding the number code.
The source for this and a whole lot of other incredibly interesting and useful information about movie posters, is learnaboutmovieposters.com