I had posted an earlier photo of this one, before cleaning all the soot, rust and wire rubs from it with oxalic acid. The seller thought it may have been porcelain, but it is definitely some form of white (though not milk) glass. The embossing shows up clearly, with this particular exposure and contrast setting. The air-mail shipping from Belarus cost more than four times ($38) the actual price ($9) of the insulator... There is no obvious manufacturer's name or marking.