snap shut plug cuffs

I've got a pair of plug handcuffs which rather than screw shut these snap shut. I can't remember ever seeing this method of locking before on a plug style cuff. Is this a common method of locking this style and I've just overlooked it ?

Thanks.

Very interesting cuff! I have never seen a Plug cuff that actually snaps shut. Curious to see if any other collectors have?
-Richard Knip-

That is very interesting....

I do have a photo of a similar one that is/was in the Museum of Lincoln Life. It would have featured in Alex Nichols book Volume 3. Unfortunately as we all know Alex died about 18 months ago and all the data he wrote has been lost. I do have all his photos etc so will try and find it.
Are you in the UK? Are there any makers marks on it?

Curious. It is still a fluted key which must engage a nut, which probably pushes and releases a spring or barb?????
It must be more complex than it looks and difficult to make/repair.

atp. can you describe the function of the locking and what the key actually does so no-one has to guess?

Hi, Thank you for your input.
To unlock the cuffs the key is screwed in to the lock tube just like a regular plug cuff. I'll just mention that it's a left hand thread.
It then differs from the norm as you don't have to turn the key in the opposite direction to engage the captive nut. 
You push the key down and rotate counter clockwise where the key then engages a solid disc / plug with four small castellations which are at 90 degrees to each other and are located at the edge of the solid plug. These castellations engage with the base of the key.
When the key engages with the plug and is turned, the plug  rotates and forces a  flat spring backwards into a recess inside the length of the lock tube. 
Considerable force is needed to compress the spring that has a "barb" on the end which engages with the notch on the post part of the other side of the shackle.
In theory anything that could engage with the castellation on the plug / disc could unlock it. However because the castellation are on the outer edge of the plug the threaded keyway gives some protection against this as anything of a small enough diameter to bypass the treated keyway is too narrow to engage the castellations.
Hope this makes some sort of sense.
ATP

Thanks ATP.  In a way it is a similar function to the Wisner Hoyt. Well described.