3 7/8" x 3 1/8" (98mm X 81mm)   (around top) AV80 3.. 24   Picture courtesy Matt Whitlock

3 7/8" x 3 1/8" (98mm X 81mm)   (around top) AV80 3.. 17   Picture courtesy Matt Whitlock

This is a spool insulator which These were typically used to dead-end a power or communication line.

 Original CD assignment from Glass Insulators From Outside North America published in 1993 by Marilyn Albers & N.R. Woodward.