No one knows???... 
This explains it for you:
"Until mid-1977, the NSS number consisted of two digits, then a slash (/), and one to four numbers. The first two numbers indicated the year of the release, the slash was a divider, and the last four digits represented the sequential order of the movie for that year. For example, an NSS number of 65/100 indicated that the movie was released in 1965, and was the 100th movie title coded by NSS for the year 1965.
When the change was made, the same numeric breakdown was used, but the slash (/) was eliminated. The first two digits of the number represent the year the poster is released. The last digits represent the sequential order of the release for the particular year. NOTICE: on the right showing 2 posters from 1977 - Greased Lighting (on the top) had the old number system while Alice Sweet Alice used the new number system.
The NSS number is usually found on the bottom border of the one sheet poster, normally on the right side, but occasionally found in the lower left corner. The NSS numbers were in different locations on other poster sizes."
