Here's a photo of the end of [id=150399645;the piece]. Notice the ground edge around the smooth interior. The ends also appear to have been ground and polished smooth. Very high quality glass.
Alrighty, all! I had just one response, but it seems to be a good one to me, and from a knowledgeable fellow! Mr Dave Kingston believes that this is a "Panel Bushing" for an electrical panel - sort of like a glass washer through which wires are fed into such a panel. The threading would correspond with a metal nut which would screw onto the bushing in order to hold it in place in the panel. Thank you, Dave!