Ephesians 3:11 This is according to the eternal purpose that he formed in connection with the Christ, Jesus our Lord, by means of whom we have this freeness of speech and free access with confidence through our faith in him. (NWT)
I think is a key scripture regarding 1975.
SimonSays mentions the book Life Everlasting and I remember discussing this with an elder some years ago when he came to discuss something i'd said. The book published in 1974 God's "Eternal Purpose" Now Triumphing For Man's Good on page 9 mentions Ephesians 3:11 and how in the Emphatic Diaglott it reads "according to a plan of the ages, which he formed" Based on this different translation the Watchtower of 1881 published an article The Plan of the Ages and gave and explanation of the Chart of the Ages evidently a similar chart was in the book The Divine Plan of the Ages published in 1886.
There is I believe a connection therefore between this book in 1974 with the words Eternal Purpose in the title and the book in 1886 with the words Divine Plan in.
The book in 1974 I don't believe mentions the date 1975 however it does talk about the 7th creative day and that Armageddon will not be part of the last thousand years of the 7th creative day but will occur before it and that 6000 years of the 7000 years had almost expired.
page 131
"The first half or 'evening' period of God's seventh creative 'day' was now closing, 3,500 years from the creation of Adam and Eve. The morning of this creative 'day' was due to begin at 526 B.C.E."
This was under the heading, "Morning of the seventh Creative day begins 526 B.C.E."
Page 51
"Evening of the Seventh Creative Day begins 4026 B.C.E."
Page 53
"At the beginning of this 'seventh day' almost six thousand years ago God 'proceeded to bless the seventh day and make it sacred.'"
The last chapter of the book is "Making the Seventh Creative 'Day' Sacred" page 187.
on page 189 talking about Satan the Devil it reads,
"For six thousand years he and his seed have been allowed to continue their effort to make it seem to be a cursed day unsacred... During the thousand years that the Great Serpent and his demon seed are abyssed, Jehovah will reverse all the wickedness."
Page 190
"...God's seventh creative day will end up blessed, sacred, sanctified. Jehovah having blessed that day six thousand years ago..."
Page 186"With all his earthly seed destroyed at Har-Magedon,"
it then asks the question "Are the original serpent and his invisible demonic seed to be left on the loose in the vicinity of our earth down to which they have been ousted from heaven? No!""By removing the Serpent and his demons from the nieghborhood of the earth and hurling them into the abyss and sealing them up there, bound as with chains for the next thousand years. Revelation 20:1-3 pictures this, not as a part of the war at Har-magedon, but as a follow up of that war."
In the Watchtower publication God's Kingdom of A Thousand Years Has Approached pages 206-211 it tells how the editor and publisher of Zion's Watch Tower and Herald of Christ's Presence calculated the presence of parousia of the heavenly Bridegroom began in the year 1874 based on the six thousand years since Adam and Eve sinned.
On page 209 it says that this was changed in 1943 in the book The Truth Shall Make You Free and this moved the six thousand years into the 1970's
So 1975 arrived at by the six thousand years calculation seems to have been the second time that the calculation of six thousand years had led to wrong conclusions. I think if one takes the dates given in the book and does the maths calculations with that Armageddon is not part of the last one thousand years of Christ's reign and the fact the book was published in 1974 in my opinion it is difficult to come to any other conclusion than that Armageddon must occur by the autumn of 1975 even though the date may not be given.