This insulator is 3 7/8" high and 2 7/8 wide. The top cap is 1 7/8". The threads continue half way into the cap and are glazed all the way up with no indication of it possibly being capped or cut with another insulator piece. The glaze transitions smoothly into the suppected wire groove. No U-number even comes close to matching it which makes me wonder if this was a patent piece that never took off or just a failed attempt. If any one can shed some light on this piece I would greatly appreciate it. Thanks