I was there. It is true. I think it came out first at an assembly in 1967. Then the date was published in a publication called Life Everlasting in Freedom of the Sons of God or something like that - a book with a red cover. Then in about 1968 the Watchtower published a study article entitled, "Are You Looking Forward to 1975?" Then in 1969, there was a special issue of the Awake! magazine called Is It Later than You Think? The Truth book came out about that time because time was of the essence and we were to have Bible studies for six months and force the householder to decide whether they wanted to be a JW and move on to others if they did not. After all, the time was short.
That is VERY interesting! I was always told it was never in print but from the platform it was "implied". I was too young in 1975 to even know what they all were talking about.