Below is pictured a prototype handcuff I invented and designed in 2007 for a well known US maker/distributor of handcuffs.
I cannot release many more details right now but I will say that the new cuff is slated to go into production in the coming year.
There are permanently attatched "sliders" that move to either side on the cuff body and block the keyholes.
The insertion of a Martin link from a belly chain ensures the sliders will not move back and expose the keyway.
This is a high security transport cuff.
Enjoy, Steve Santini