Here's a photo of the strange milkglass porcelain composition insulator. Mine is identical to the obvious glass examples except that it is not transparent. I initially thought the surface had an odd texture to it but after assessing it today I'm confident it is identical to glass and has no surface coating to it.

This example rings like a beautiful bell made of extremely high quality porcelain! Much more than any of the glass models in my collection. So perhaps this is proof it is porcelain? I have owned Russian made brown glazed porcelain examples in a larger size but similar shape as this insulator and they were much heavier proportionately than glass counterparts. In other words, this milkglass example has the expected weight and texture of glass but the ring and of porcelain. Of course, high quality glass will also ring like high quality porcelain.