Looks like an insulator, but its got notches encircling the outer ridge and a cemented threaded bolt on the inner base. I've seen one before on eBay but don't think it was identified there either. Measures 5 1/8" wide x 3 inches tall. Looks like something Ohio Brass made based on the dark chocolate glaze.

Update: Robin Harrison added the following details- I think it's probably used for electric lamp hangers. There would have been a clamp with a loop for suspending the insulator around the knobby top and the fixtures would screw onto the bolt underneath. It's possible the next hardware down would have been a bar with insulators on each end to hold the lamp power wires.

Jimmy Burns added: Westinghouse produced similar street light heads as did GE in the 1940's. These are smaller have metal collar on top. I have one in my collection. Also have a GE catalogue that illustrates it.

Thanks for your help!