Clear glass square electrical insulator?

Bought a house at a tax auction. Buffalo, New York, USA. House is full of stuff. Mostly trash. Found this glass thing. What is this?

Material is glass. Weight is 836.5 grams, little under 2 pounds. Recess on the bottom is about 1 - 1/2 inches deep. Grove is only on one side. There are 10 marks on the top. There is one mark on the side that has the grove. This mark is shaped like this ( rotate clockwise, sad face.