The additional deadbolt provides a second, independent lock on modified 856 or 859 door locks, increasing security for the end user. The modified locks have a slot in the faceplate to allow the bolt to be thrown and locate in an additional hook plate.

The lock fits above the handle at a comfortable height for ease of use. It can be operated internally by key or thumb turn, and locked externally by key. Alternatively a half cylinder can be used for internal or external locking depending upon the security requirements of the end user.