Recent trip I picked up this insulator at an antique mall sitting on the shelf of an England-imported furniture piece. Double wire groove, double petticoat, 4 inches tall, made of a black stoneware or composite material. Recessed embossed "T5" on the dome, very small diameter and deeply recessed pinhole. Looks english to me, but other than that who knows something?

Ans: A termination insulator used in England and composed of a vandal-resistant composite material