I recently bought this EIV HT-21 2 part glass power insulator from "Open-Wire" . Aqua-blue glass, two shells, Notice the nicely bell-shaped lower section! Vintage appears to be fairly recent, 1980s or 1990s. I took the liberty of using his supplied photos (as they wouldn't look as good sitting in an inch and a half of NC snow).

"HT" stands for "Haute Tension" which means "High Voltage" in French.