Here is a clear view of how CD 280 or similar "mushroom" insulators (including porcelain and composite) were used as a joint between the end of a gooseneck street light bracket and the fixture housing. The latter had metal screw threads atop it to accept the insulator as it was clamped around its wire groove to the end of the mounting pipe. In addition to a physical connection this design provided electrical insulation between the fixture and bracket.

Also please note that provision was made for two small pintype insulators was made on the fixture bracket. These provided dead-end connections from above.. and then connecting with leads down to the fixture socket.