Cult members are only allowed to believe what their leaders tell them to believe.
So, ask yourself this question: Why did JWs believe 1975 would mark the End of the World? (We know they did believe this, because a large percentage of them left the organization after 1975 came and went.)
Did they get this 1975 idea from a religious source outside of the WT organization? Of course not. They're not allowed to read outside literature. Were they attending church services somewhere other then the Kingdom Hall and JW assemblies and conventions? Of course not, this is forbidden.
So, where did they get this crazy idea about 1975?
To me, the answer is self-evident. The idea came straight from Brooklyn.