What may appear to be a standard model 104 Darby handcuff is actually a rare variation which uses an externally threaded key and an internally threaded lock tube, vs the normal internally threaded key which threads into a spring loaded bolt.
HIATT 3 makers mark
Above close up of the "male" key, known as Sheffield
I have no idea the exact interior workings of this type of Darby lock. I would imagine the locking bolt is replaced by an internally threaded sleeve which otherwise acts the same as the locking bolt, I.e. spring loaded snap shut, but I know that KUB Sheffield models did not snap shut, so I'm not sure. I would imagine that this would need to have fairly stiff springs because otherwise any bolt of the appropriate major diameter could be forced in and cross threaded enough to pull back on.
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