That 1975 being the end of 6000 years of human history was first mentioned in a publication that was released at an assembly in 1969 .i think it was called "Life everlasting In freedom Of The Sons of God" if memory serves me correct.
And it was suggested there that it would be appropriate for Jesus to then begin his thousand reign.
What the average JW does not know is that 1975 was not the first date the GB had said that 6000 years would end in 1975.
Somewhere in their earlier publications you will find they believed and taught a much earlier date was the end of 6000 yrs of human history. And with new light , they had to make an adjustment to come up with 1975.
If memory serves me correct it had to do with someone called " Usher" chronology" ? being so many years out , maybe 40 yrs ? or so.?