The streetlight you see in the center of this photo was commonplace as residential incandescents through the teens. They basically operated on an electrically-operated-series circuit like a string of current-day mini-Christmas lights. When one lamp failed, the remainder stayed lit (or were supposed to!). The fixture shown had a CD 280 or similar style (also in porcelain) affixed between the light shade and gooseneck pipe arm. The light socket was placed in a metal cap at the base of the insulator pin. Two wires led to the fixture and typically a porcelain guy wire breaker or transposition insulator terminated the circuit on both sides at the crossarm whereby each of the two series wires terminated at the fixture...on two white porcelain knobs. I'm always seeking parts, literature and anything else pertinent to these types of lights. Thanks and enjoy!