Typically a glass CD 280 or its porcelain equivalent (especially those by those made by Pittsburg High Voltage Porcelain) were utilized as a coupling bewteen the top of fixtures like this and the end of its pipe-design gooseneck bracket. Such insulators provided electrical safety from possibly inherent high street lighting voltages as well as making a physical fixture connection with its mounting bracket.

Hint, hint...I collect this kind of stuff and am looking for any parts, reflectors, couplings, etc., like used with the fixture you see here, thanks!