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Cuttle bone comes from cuttle fish: https://en.wikipedia.org/wiki/Cuttlefish

It is sold through the pet trade for pet birds to use to groom their beaks.

If your local grocery has a pet section they may have it on display.

Bolt/nut in this position because it is the way the come from the factory.

Evidently previous owner never touched it.

If saddle soft it could probably be taken up a bit to preserve tension; you do not want to stretch the top, just take up any looseness.

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