Mullard describe the 90CG as a gas filled photo cell, most sensitive to incandescent light sources and to infra-red radiation.
The maximum anode voltage is 90 V and pin four faces the direction of the incident light.
Caesium on oxidised silver is the active material and the cathode effective area is 3 sq cm. The dark current is 0.1 µA and the maximum current is 2 µA.
The cathode is shaped as a half cylinder.
The anode is a single vertical rod.
Thanks to Stig Comstedt for sending the data-sheet.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 47 mm tall.
Reference: Data-sheet. Type 90CG was first introduced in 1955. See also1955 adverts.