The Humble Insulator

A true insulator is a material that is unresponsive to an electrical field and totally resists the flow of electric current. In application these are non-existent. What we have is dielectric. Materials with a high dielectric constant is typically what we call an insulator. In very old wiring what you'll find are ceramic insulators. This one is from my basement and clearly porcelain, material made from clay, alumina and feldspar. Porcelain has a dielectric strength of about 4–10 kV/mm, it's inferior to glass but they were easier to mass manufacture. they were also made of white flint, but those are more rare. More here.

If your house is more than about 50 years old you might find some of them up in the rafters of the basement or attic.You can see that this model is simple, with two parts with a center aperture for a nail to both mount it to a beam and also clamp the two parts of insulator together securing the wires. The two grooves tell me that it was meant to hold two wires. It has no markings so I can't tell you any more about it.

I found a similar bunch here.