The explanation about the indefinite period between Adam and Eve's creation was well discussed before the release of "Life Everlasting in Freedom of the Sons of God" in 1966 and the subsequent hype about 1975. Even the book "All Scripture Is Inspired of God and Beneficial" published in 1963 has a thorough discussion of this topic in the chapter entitled "Chronology." (I no longer have a copy or I would give you the page and paragraph number.)
This whole matter was a subject of discussion in my congregation in Washington state prior to the 1966 conventions. My uncle, Russell Poggensee, returned from a refresher KM School course barely able to contain himself about the new light. He sort of botched its presentation though. All he could say was that the Society had figured out exactly when 6,000 years of human history would end and the thousand year reign begin. Others in the congregation found the above information in the SI book and refuted his claims.
I'm sure my uncle felt vindicated post '66 convention, but the arguments still stood. What surprises me the most is that many who knew the '75 prediction was bogus are still witnesses.