This insulator is used at the lower end of an arrangement to bring a pair of open wires down a high building to a lower point where they can terminate on the lead-in cable at a convenient height for fault finding. The upper point uses the Insulator No. 10 also with side knobs.
Unfortunately these specimens and their bracket have been painted to match the woodwork.
The insulator measures 2¾″ in diameter and 4½″ high. The top recess for the connectors, under the screw cap is ½″ deep and 1¾″ diameter. Two ⅜″ holes drop out of the recess for the terminating cable(s). Just above the top groove is a single ⅛″ hole for the copper wire to enter the recess. These dimensions are the same as the No.16, but this insulator only has a single main groove. The knob is 1⅝″ in diameter, sticking out ⅞″