This side knob insulator is used at the top 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 lower point uses a pair of Insulator No. 21 a pot head insulator with side knobs.
Unfortunately the lower specimens and their bracket have been painted to match the surrounding woodwork.
RingBell : GPO Nostalgia Web site