As far as Watchtower theology goes, the first appearance of this teaching came in the 1966 release Life Everlasting in the Freedom of Sons of God. That was the first time the glorious new light was shared.
Which teaching do you mean? 1975? If so, that is not quite correct.
The 1943 book "The Truth Shall Make You Free" pointed at 1972.
" ... From Adam's creation to the end of B .C. 1 was 4,028 years
From beginning of A .D. 1 to the end of 1943 is 1,943 years
From Adam's creation to the end of 1943 A.D. is 5,971 years
From beginning of A .D. 1 to the end of 1943 is 1,943 years
From Adam's creation to the end of 1943 A.D. is 5,971 years
We are therefore near the end of six thousand years of human history, with conditions upon us and tremendous events at hand foreshadowed by those of Noah's day .-Luke 17 : 26-30."
-- "The Truth Shall Make You Free," chapter XI "The Count of Time" page 152
-- "The Truth Shall Make You Free," chapter XI "The Count of Time" page 152
If 1943 was 5,971 years from Adam's creation, then 29 years later - 1972 - would be the 6,000th year.
Certainly the "brothers" in 1943 were able to do the math.