MadApostate - Used this on my dad.
At first my dad saif the society never said it. Then I pulled his bound voume of the 1968 Awake! and went to the article "What do the 1970's mean to you?" (or something like that.) Then he admited it, but said it was just a writer's opinion. Then I reminded him the Governing Body wrote the articles. Then and only then did my father admit they said 1975.
I think all the elders know, my dad sure did, but he was taking part in "Theocratic Warfair," the term used when it's OK to lie to protect the Wt unless you are caught with your pants at your ankles.
If you can get your hands on that Awake or the bound volume, you are set!