This is a special black box which we have just imported from the US.  A more detailed description will follow but these clever boxes prevent access to a pair of handcuffs when locked into position.  They were invented by prison officers 3 generations ago and the same family still sell them

Here is a link to the C and S Security video, explaining how the system works

https://www.youtube.com/watch?v=O_4xnzwyg40