The Insulator No.12 has two grooves whereas the No.11 has a single groove. Made of brown earthenware or white porcelain, this type of insulator was used when a pair of wires passed through but didn't terminate. In the top right example the upper pair of wires would have terminated here and the lower pair passed by.
Locally there are many examples of single insulators screwed onto spindles carrying a dropwire clamp. I am guessing these were used in lieu of a bracket No.22 for dropwires.
RingBell : GPO Nostalgia Web site