I have confirmed on CD 186.2 and CD 186.1 (previous picture) that these exist in two sizes, the "normal" height, and one with an intentional 3/8" skirt extension. The reason for this is as much, or as more, of a mystery than the function of these pieces.

Nothing has been known about the function of these pieces. There is a six sided machine nut with fine threading in the dome, and a metal thimble in the pinhole, with perhaps 1/4" space between the two.

An ex-Hemingray employee has a machine drawing of this piece, and it is labeled "Spark Gap Asserter". Nothing more. Given that description and the construction of this insulator, my guess is some device which held the conductor was screwed into the nut. The insulator, with it's metal thimble, was mounted on a grounded pin.

As I mentioned, there is a 1/4" gap between the nut and the thimble. Perhaps when excessive voltage was flowing through the wire, it would arc down to ground, and protect some device? You decide from the name of the piece and it's construction.