A833 Neutral Issue

I bought this trans years ago with intentions of using it, but never have. I believe it came from a 69 GTS car. With the levers in the neutral positions, the input spins the output shaft basically at a 1:1. I've pulled the side panel and no chunks, shavings or missing teeth. Everything is really clean with visible paint markings on some of the gears. If you move any of the shift levers, it locks up the trans. Thoughts on what it could be?

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