I thought that the 1975 thing started in the mid to late 60's, but it seems the seeds were sown two decades earlier.
Is this the earliest reference to it?
w44 9/15 p.280 par.28
18 How many today are awake to the fact that the Lord Christ Jesus, by his resurrection of the faithful bride members that slept in death, is taking his "bride" unto his Father's house and the marriage of the Lamb has begun' ...
They do not say, 'It is a long time yet to the wind-up, and, according to the present understanding of Bible chronology, six thousand years of human history will end first in the coming seventies and till then it is a long stretch to serve the Lord continuously and undividedly.'
... They keep their eye and heart on Jehovah's kingdom, and not on the time clock. It The present is the time to serve the Lord, while your individual opportunity is here! Who guaranteed to you that you individually will last till the seventies or even till tomorrow'.
... Hence make the most of your own present. Do not take chances and say, 'According to Bible chronology it will be some years yet, and I shall have time to get into Jehovah's royal service after I have grown some years older. Therefore I will use the present time for going after the same things as do the Gentiles who live for the present. But in the distant tomorrow, or in the final cubit of my life-measure before the battle of Armageddon, ah, then I shall get in real earnest and give my full time, strength and means to the Lord's service.'