This is the porcelain SURGE. The metal bracket is molded into the insulator. The insulator just seems to swing loose from the hole in the bracket, with the line wire attached to the insulator. This seems to show a photo of them:

http://www.glassian.org/Babson/Handbook/page4.html

You can also see a picture of this in the [id=149294265; 1941 Surge handbook].