Knightsbridge FP7400BB Flat Plate Telephone Extension Socket - Brushed BrassKnightsbridge FP7400BB Flat Plate Telephone Extension Socket Brushed Brass Secondary or extension telephone socket are used to run additional points of the original master socket. Standard BT type socket with screw type terminals. Flat Plate low profile design Extension BT type socket Screw terminals Ultra low profile design with built in gasket Certified to BS 5733 Minimum mounting box depth: 25mm