I include this next bit for the benefit of some like Debator et al who look at the letter of what was printed and conclude and argue the issue of whether or not the society said 1975 was it. And while I can acknowledge it is difficult to find the definitive quote to prove they did I can assure that by innuendo they did indeed convey that those were the last days of this system - I was there.
I actually came across the info in the intro of this thread looking up the book THE TRUTH THAT LEADS TO ETERNAL LIFE
And why I was looking up that book is because I remember how the message in that book about the "six month study program" affected me. And this was in 1968. How it affected me could probably be expressed using the word mesmerized. I can remember where I was sitting and listening to this info because while not coming out and saying it necessarily in so many words the clear inference was that the time was so short that we could not dilly dally with bible studies who would not commit and act on what we were teaching them. This is how it was expressed:
***jvchap.8p.105DeclaringtheGoodNewsWithoutLetup(1942-1975)***
This new publication was to be used in connection with a six-month Bible study program. The September 1968 issue of the KingdomMinistry explained how the new program would work: "It would be good to try to study awholechapterofthe'Truth'bookeachweek, though this may not be possible with all householders or with all the chapters in the book. . . . If, at the end of six months of intensive study and conscientious efforts to get them to meetings, they are not yet associating with the congregation, then it may be best to use your time to study with someone else who really wants to learn the truth and make progress. Make it your goal to present the good news on Bible studies in such a way that interested ones will act within six months!"