thats a very rare insulator, youll be lucky if you ever get to see one. They used some of those on the central georgia railway. Here in ohio, they used different boston bottle works insulators on the mad river railroad, it might run near you actually. Just think, so close to $1000 insulators and yet so far.
Those are neat insulators. The threaded protrusion on the top was for a wooden cap that secured a lead off wire. A square metal pin with a groove for the lead off wire was inserted through the square holes in the top of the insulator, then the wire was pushed through on top of the pin and the wooden cap was screwed down tight to secure the wire. The free end of the wire was swedged to the exsisting wire and the lead off wire was pulled to the next pole.
Ahhhh, one of the many nearly useless facts nestled away in the dark reaches of my pea sized brain. [sm=tongue.gif]