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I've never looked that closely but I'm not sure the shafts are even as good as fibre glass, more like a translucent nylon type plastic, they certainly have some flex. I've never had one break but they've aways seem rather weak. No, there's no special requirement, other than being non conductive, mainly to avoid accidental shorts against PCB components. If you can get the rod cheaply then there's little point in spending over the odds for an original, spend it on a decent condition replacement knob instead.