A few ICONners knew exactly what the porcelain thing was: Lou Hall, Rick Soller, Lee Southern. It is a counterweight for a hanging light fixture. Like insulators, they come in different shapes, materials and some can be quite ornate.
Here is a great description provided by Lou Hall.
It's a counter weight for vintage pull down light fixtures. The bulbous portion was filled with bb's (shot) to match the weight of the fixture so the fixture could be raised or lowered to the height desired.