More than likely it's a single action compression rod similar to what Gibson and Fender uses. There's a good chance it is not robust enough to flatten the bow. Use a couple of blocks, a board, and a couple of clamps to force the bow out of the neck. That will take the load off the truss rod. Once the neck is straight, tighten the truss rod. The nut should turn easily. 90° is not much on this type of rod, it may take more than a full turn. The rod will probably hold the neck in place, it just needs a little help getting there.
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Rodger Knox, PE
1917 Martin 0-28
1956 Gibson J-50
et al
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