They did not write that Armageddon would occur in 1975. They did write that the Millennial Reign must begin in 1975...
*** w68 5/1 pp. 271-272 Making Wise Use of the Remaining Time ***
5 Therefore, God’s s event h day and the time man has been on earth apparently run parallel. To calculate where man is in the stream of time relative to God’s seventh day of 7,000 years, we need to determine how long a time has elapsed from the year of Adam and Eve’s creation in 4026 B.C.E. From the autumn of that year to the autumn of 1 B.C.E., there would be 4,025 years. From the autumn of 1 B.C.E. to the autumn of 1 C.E. is one year (there was no zero year). From the autumn of 1 C.E. to the autumn of 1967 is a total of 1,966 years. Adding 4,025 and 1 and 1,966, we get 5,992 years from the autumn of 4026 B.C.E. to the autumn of 1967. Thus, eight years remain to account for a full 6,000 years of the seventh day. Eight years from the autumn of 1967 would bring us to the autumn of 1975, fully 6,000 years into God’s seventh day, his rest day. [...] 7 The immediate future is certain to be filled with climactic events, for this old system is nearing its complete end. Within a few years at most the final parts of Bible prophecy relative to these “last days” will undergo fulfillment, resulting in the liberation of surviving mankind into Christ’s glorious 1,000-year reign. What difficult days, but, at the same time, what grand days are just ahead!
*** w70 9/1 p. 539 Preparing for the Approaching Millennial Peace ***
Further, Bible chronology indicates that we are at the close of six thousand years of human history. For Jesus to be “Lord of the sabbath,” his thousand-year reign would have to be the seventh in a series of thousand-year periods. Would not, then, the end of six millenniums of mankind’s laborious enslavement under Satan the Devil be the fitting time for Jehovah God to usher in a sabbath millennium for all mankind?