Three pins for comparison. The one to the right is a common eight inch peg with 1.25 inch base.

Peg on the left is from a high voltage crossarm, also eight inch butt fits in a 1.5 inch hole.

Centre peg is a mystery. Exactly eight inches, also fits in a 1.5 inch hole, . Two inches of tapered threading 5/8 - 1 inch' [id=196021168]

All three pins are four threads to the inch.

The threads are weathered and somewhat elliptical but a surge insulator does thread on quite easily for a couple turns but does not thread on as far as it should because of the taper. The Surge side pins Bill measured are 3/4" in diameter with no taper and have about 1 1/2" (or less) of threads. If you look at Lou's pin, [ id=195946020] it is threaded for less than an inch .

I have received information of one other example known. This pin does not fit any insulators that I have tried. CD 100, CD 143 double threaded, or CD 718. I have tried many foreign pieces as well.