I say during the 70's & 80's in the last part of F Franz's time as oracle and when R Franz and some others seemed to be seeking real truth. In the 80's, it seems that R Franz and some other JW higher-ups were using Bible commentaries, etc. to seek better understanding. The org got scared because it realized real truth would expose its doctrines. A witch hunt ensued, and since then the org has frowned on JWs' using non-JW material to do study and research. It doesn't want JWs to learn Bible languages. The org is now not a lover of truth, but a fearer of truth and open knowledge.
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I get the point of what you're asking, but, even though I'm going against your request, I have to add that, now, looking back, I don't think the org ever had "the spirit". In my answer above, I'm just indicating when a major change took place. Before that time, the org seemed honest, sincere, bold, and even somewhat scholarly. The doctrine, to me, had the ring of truth. That's why it appealed to me and the many smarter, more level-headed JWs I knew at that time.
Now, JWdom is a joke. It would be unrecognizable to many of the ones I knew a few decades ago who haven't seen it in the last twenty years. JWs run from questions. The org is no longer bold; it's puny & weak. It realizes it has lost in the doctrinal arena and is playing it safe now with dumbed down literature.