ding: The date was announced in 1966 in Life Everlasting in Freedom of the Sons of God, so I doubt it had anything to do with a tax exemption in 1971 or 1974.
Do you have information that confirms that the WTS was not under investigation concerning their tax status in 1966 or in the years leading up to that? Are you aware of what was happening with non-profit tax law at that time?
Did you read the pdfs I linked to?
The struggle to maintain tax status did not just happen overnight. There was a whole list of corporations that lost tax status in 1971. I am sure that the IRS didn't just wake up on January 1st 1971 and say...let's take away property tax concessions to all these nonprofits.
It is a process and one that can take years. The WTS would have known what was in the works for years before it actually happened