Good question. I don't know, but perhaps an override key. However now that the physical key isn't an open secret, and needed by everyone, they would be easier to secure. And small batches would have a specific key for that batch, but not for all cuffs, making it even harder for the detainee to have the right duplicate on hand.

I feel though that my larger point is getting lost in this thread. It was/is:

Just because a security technology doesn't work for reason $x in situation $y, that doesn't mean that the weaknesses also apply to situation $z.