This insulator was one of three fixed to an old pole transformer box. The top section down to the first flange is fixed inside the box via a lot of bitumen and a bolt which runs through the centre, poking out of the bottom where a brass nut holds the whole assembly in place. The power wire is then affixed to the remaining stub of thread.
A nice clear printed mark graces the second shed.BULLERS ENGLAND 1-73 864
I assume this represents the date of manufacture, January 1973 and a pattern number 864.
Overal dimensions are 7.1/4 inches high and 5.3/4 inches across the widest point.