The missing link

Taken over 50 years to find these. I believe they are the missing link between the Griffin Solid Centre cuffs and the Plug 8 type cuffs. They are superbly made and the tolerances are wonderful. I can not find a makers name at the moment but in the same lot was a Bilboe Handcuff which I could not believe is stamped HIATT!!
Any thoughts?

Photo of the Hiatt stamping.

Amazing!!!
The Plug Bar Cuff is unique. The spiked bolt is similar to Boer War cuffs and some early plug leg irons. It would require a key that's drilled to accommodate the spike. As it is plated, I'd date it after Griffin Plug 8s and as the plug, nut and bolt mechanism was tried and proven it's possibly an attempt to make a more robust plug 8. The other bar cuffs with small key are based on locks from something like a box/chest. Nichols was a locksmith primarily, before teaming with Griffin as a maker of restraints and so there would be some overlap of skills and experimentation in designs. I would expect all these weird designs including the "side snap 8" to have been made in the period 1810-1840 when designs were finally settled on - being standard 'darby' and 'plug" cuffs and irons.
The Hiatt marked bilbo handcuff is the only one i've seen marked as Hiatt. Such a simple cuff could be easily made by any blacksmith. 
As Hiatt was known to 'make to order', it was probably made for convicts to Australia or for the slave trade. But anyone who didn't want to mess around with keys might prefer them.

Thanks Paul for your thoughts. I have finally found the photos of another one that is similar in a Museum at Saffron Waldron. It is dated as early 19th century. You can see it probably started as the Solid Bar Cuff as you can see the old keyway.

Well now I've seen two!
The time period ties in with my thoughts, as does the theory of the bolt/nut/plug lock replacing the keyed box lock. 
So the old keyway remains, but the alteration was extensive if opening from the end now instead of from the centre.
The introduction of Police constables around 1840 would/could have created a need for more handcuffs and possibly the manufacture of locally made restraints?

A fantastic find!