Pretty much all cells here (except mini cells built into luminaires) were like this :
http://www.lighting-gallery.net/gallery/displayimage.php?album=2054&pos=10&pid=57693So the bulkhead was the most common installation. Occasionally a transparent cover rectangular electric box was used instead. The bulkhead itself was usually this one :
http://www.electrosen.co.il/products/פיטינג-מנורה-מוגנת/(Its now they list it for 18W CFLs, but origianlly it is rated for 40W/60W GLS, which is probably still printed in the Bakelite luminaire base itself unless they blanked it out). Inside is E27 socket on angle bracket. If that is removed, the photocell with its bracket can be fitted to the same screw hole
Virtually identical bulkhead is made today by 1 other manufacturer, back then was made by 1 more, so there were used bulkheads from all 3 of them