I recently acquired this insulator from Australia via Ebay purely for its link to Bullers.
As with many Bullers insulators, it shares aspects of other Bullers made insulators. The top is reminisicent of a Langdon and the underneath is internally similar to a double shedded langdon. I am sure many of the same profile tools were used across the insulator range.
It is internally threaded with the standard Cordeau thread and the inside rim of the outer shed is impressed with the number 25.
Stamped Bullers Ltd London on its large outer shed, it measures 4.3/8 inch at its widest point and 4.3/4 inches high, altogether quite a hefty insulator.
Update - December 2005
A small variant of this insulator came to light while visiting Jim Gough. It measures approximately 2.5 inches in height