Revised 5/1/2017

The National Screen Service (usually referred to simply as the "NSS") was a company that started in the mid-1920s, and which provided film trailers to movie theaters (prior to film trailers, theaters used glass slides to project a single color image of an upcoming movie onto the screen between showings, and the film trailers were a HUGE improvement over that, because you could actually see highlights from the movies themselves!
     Up until the late 1930s, each movie studio would distribute the posters for their movies themselves, and this meant that every studio had to maintain a small warehouse in each major city where those posters were stored. Sometime around 1937, some genius at NSS came up with the idea of them handling poster distribution for ALL the studios, so that there could be just a single warehouse in each city!
     It must have taken a couple of years to put this massive plan into operation, and it appears the studios began supplying NSS with posters in 1937 (even though they would not have numbers printed on them until 1940) because almost all NSS exchanges had posters dating back that far in them (when collectors began buying the posters from the exchanges in the 1960s).
     The numbers started in 1940. For the first two years the NSS numbers were a "40" or a "41" followed by one or two more numbers, and then a slash mark, followed by more numbers, like "4017/57" or 4113/62". Starting in 1942, the numbers were two digits (signifying the last two digits of the year) followed by a slash mark and more digits (like "42/29"). Sometimes the NSS numbers (and the rest of the copyright info) was "handwritten" and sometimes it was machine set (I imagine it mostly depended on the studio).
     Not all films were distributed through this NSS program. Many small companies did not bother to get involved, especially those that imported foreign films. These films often were dubbed and given English language names on the posters. Generally, most of the independent (non-major studio) films and adult films do not have NSS numbers printed on them.
     Also, the major studios did not all join this NSS program right away in 1940, and it took a number of years until almost all one-sheet posters from major studios would have NSS numbers on them (Columbia was the last to join, and that was in the late 1950s).
     NOTE THAT THE PURPOSE OF THE NUMBERS WAS TO MAKE QUICKLY FINDING THE POSTERS FAR EASIER! The people who ran the NSS warehouses realized they would soon have tens of thousands of posters from hundreds of movies (later hundreds of thousands from thousands of movies!) They realized that finding a, say, This Gun For Hire poster would be FAR easier if they could simply say, "Give me a 42/174" and the employee would go to the 1942 shelf and go down to number 174!
    NOTE TOO THAT THE SLASH MARK THEY INSERTED (so employees could know the year) HAS CAUSED A MASSIVE AMOUNT OF CONFUSION OVER THE YEARS! This is because people see "42/174" and naturally they THINK they have found a LIMITED EDITION PRINT (they think it was limited to 174 prints and that this was #42)!
     Actually, the number after the year is how many films were registered that year. So "42/174" means it came out in 1942, but what does the "174" mean? I used to think that the smaller the second number, the earlier in the year it was released, but I have learned over the years that the major studios were assigned blocks of numbers at the start of the year, and they put them on movies as they saw fit, so this was not always the case at all.
     Would this "assigning blocks of numbers to studios at the start of the year" mean that there were sometimes "unused" NSS numbers? It is possible, but I don't think so, both because I think that when a studio planned 22 films for a year, they DID almost always release that amount, although clearly some projects were "shelved" and replaced with others. Also, it is possible unused numbers were then assigned to other studios, or independent films.
     I can do much more research on this (when I find the time!) because I record EVERY single NSS number from every poster I auction (as well as litho numbers, which are another thing entirely, a subject for another time!), so one day I will look at all the NSS numbers I have recorded and see if there are any "missing" ones, and whether I can learn anything else from studying them.
     Sometime in the late 1960s, when the ratings system (G, M, R, and X, later modified to G, GP, R, and X, and modified three more times since) was added in the U.S., the studios started printing posters for the U.S. market which HAD ratings on them and also NSS numbers in the lower right, and ALSO posters WITHOUT ratings or NSS numbers for the non-U.S. market (because countries outside the U.S. did not have our rating system and they did not use the NSS numbers to file the posters). Sometimes these posters (called "international styles") would be completely identical to the U.S. posters (other than lacking the ratings and NSS numbers) and sometimes they would have completely different (often sexier) images!
     HERE IS A FUN ADDED BIT OF INFO! We recently came across two different movies (Leonard Part 6 and Someone to Watch Over Me) that have the EXACT same NSS number printed on them ("870123")! We have no idea why this was, but suspect it was human error at NSS. We also have "870110" recorded in our database for Someone to Watch Over Me which was the number that SHOULD have been printed on those posters. (it came from the NSS book, which is a wonderful book of all the numbers that were assigned to movies by NSS. We acquired it some time ago, and we kindly allowed the LAMP site to use all the info on their site, so all collectors could have access to this info. Here are images of those two posters (and close ups of the NSS numbers)!

Here is a link to a related article of interest:
Did you know... that many years ago, there were "movie poster exchanges" where lots of early collectors obtained their posters?