This is a black 1.5mm thick steel plate that is bolted between a two-bolt style towball and the towbar neck. It is for a 12N, 12S or 13-pin socket and is suitable for fitting on the left or right side of your towball. The plate is suitable for Witter, Westfalia, Tow-Trust, Bosal, Brink and other towbars.  
Other electrical socket mounting plates are also available, see our other items for sale in our shop