You are correct, of course.
In 1975 the WTB&TS said that 6,000 years of human existence were coming to a close. The implication was that this would be followed by the 1,000 year reign of Christ, and then the 7th creative day would end.
Recall that each creative day was pronounced "good" by Jehovah when it had concluded. Supposedly, we aare living in the seveenth creative "day" and it could not be pronounced "good" unless Christ's Kingdom had cleansed it of the not-good effects of sin.
CTR accepted and taught the belief that was popular in his time that each day of creation was 7,000 years long, but this idea can be traced further back in history, even to pre-christian times.