Found these while scratching around today. No markings. The damaged one was also on a pin. They measure 3-1/2" high x 3" in diameter at the base. I haven't a clue as to their purpose, maybe somebody out there can shed some light. Thanks for looking. Answer: These are pin bases, mounted underneath a regular power insulator on a special cob/pin arrangement. The purpose of the pin base was to increase the insulating value of the power insulator. Thanks to Ken Willick and Dave Dahle for their responses.