Yesterday I purchased part of a collection including this unusual insulator . There are no markings except for the mold line from the wire groove down. The shape is different. Base measures 3 1/4 inches across, height 3 1/2 inches. The pin hole is 1 3/4 across and the threads,[threaded area] is 1 inch deep. As can be seen in the picture, the wire groove is quite rusted, so there appears to have been a tie-wire. Picture 2 is a view of the pin hole

Doug Netzke [Sluggo] of Sudbury, Ontario has identified this as CD 405 from the Gifona book. Value shown as $200-250- lucky me!!! Thanks Doug