A handcuff cover is a piece of plastic or metal that can be placed around a pair of handcuffs to convert a pair of standard handcuffs into rigid handcuffs and to cover the keyholes of the handcuffs for further security.
CTS Thompson "Blue Box" handcuff cover
The handcuffs are put into the plastic box.
The box is closed and fixed with the metallic slider.
Inmate in handcuffs with a handcuff cover, secured with a belly chain around the inmate's waist.
Inmate in handcuffs with a handcuff cover, arms stacked.
Closeup of an inmate in handcuffs with a handcuff cover, front stack position.