I believe that lid came off the paint can and the painting of themselves into a corner began with the QUESTIONS FROM READERS in the September 1, 1952 WATCHTOWER magazine. On page 542 the question was posed:
"Your publications point out that the battle of Armageddon will come in this generation, and that this generation began A.D. 1914. Scripturally, how long is a generation?" - G. P., Liberia
Answer: "Webster's unabridged dictionary gives, in part, this definition of generation: 'The average lifetime of man, or the ordinary period of time at which one rank follows another, or father is succeeded by child; an age. A generation is usually taken to be about 33 years.' But the Bible is not so specific. It gives no number of years for a generation. And in Matthew 24:34, Mark 13:30 and Luke 21:32, the texts mentioning the generation the question refers to, we are not to take generation as meaning the average time for one generation to be succeeded by the next, as Webster's does in it's 33 year approximation; but rather more like Webster's first quoted definition, 'the average lifetime of man'. Three or even four generations may be living at the same time, their lives overlapping. (Ps. 78:4; 145:4) Before the Noachian flood the life span was hundreds of years. Down through the centuries since, it has varied, and even now is different in different countries. The Bible does speak of man's days as being threescore and ten or fourscore years; bit it assigns no specific number of years to a generation.-Ps.90:10
Even if it did, we could not calculate from such a figure the date of Armageddon, for the texts here under discussion do not say God's battle comes right at the end of this generation, but before it's end. To try and say how many years before it's end would be speculative. The texts merely set a limit that is sufficiently definite for all present practical purposes. Some persons living in A.D. 1914 when the series of foretold events began will also be living when the series ends with Armageddon. All the events will come within the span of a generation. There are hundreds of millions of persons living now that were living in 1914, and many millions of these persons could yet live a score or more years. Just when the lives of the majority of them will be cut short by Armageddon we cannot say."