Can I ask HOW you found it?
The March 2016 WT has a revised teaching on when Babylon the Great took God's people captive.
(For info it is now the 2nd Century CE, not 1918.)
When new light comes out, I like to go back and search for what the WT used to teach, and catalogue the flip-flops.
In this instance, a search of the WT CD ROM led to w77 10/1 where in an unrelated paragraph (p.14 par.12) it quotes from the 1944 WT but uses elipses (...) in the quote.
Knowing how WT use elipses to remove critical information, I looked up the 1944 WT (I have the complete set as PDF's) and saw the reference to the seventies in context.
To see if there were any earlier instances I scanned the PDF's from 1915-1944, and this was the first occurrence.
Simples! (squeek!)