Darby cuffs have been around for 300+ years. There is an adjustable model, but most of the ones made were not adjustable, as in this animation. The key is an internally threaded one. The action of screwing it onto the bolt causes the bolt to retract which releases the bow. Locking does not require the key since the bow can push past the bolt which then springs into a hole in the bow, locking the cuffs.