Part of the reasoning for the end coming in 1975 was that would be 6000 years after creation. The 1000 year reign would start and the end of it would coincide with the 7000 rest for God (like an omnipotent God needed to rest after creation, but that is a different topic.)
The 1966 book Life Everlasting, which presented the idea that the end would be 1975 lists in the following table that the 1000 year reign ends in 2975.
That was a neat concept prior to 1975, but since Armageddon did not come then it now makes Watchtower eschatology look quite foolish, and more so with each passing year. There are now supposedly only 966 years of Jesus 1000 year reign left. Any chance the Watchtower will start talking about Jesus 950 year reign?
For quotes on 1975 see http://www.jwfacts.com/index_files/1975.htm