The 20CV is a caesium on oxidised silver vacuum photocell manufactured by Mullard Ltd. Designed to work at an anode potential of 100 volts with a series resistor of at least 0.1 MOhms - typically 1 to 5 MOhms.
It has a B8G (Loctal) base with pin out as shown above.
The type 'C' Cs/Ag2O sensitivity is at the red end of the visible spectrum and is specified as 25 uA/lumen.The sensitivity is tailored to incandescent, i.e. tungsten filament, lamps. Maximum cathode current is 20 uA. Anode to cathode capacitance is 1.2 pF. Frequency response extends to about 10 kc/s (kHz.) where sensitivity is down by about 20%. The type code 20CV indicates: Loctal (2); serial (0); caesium on oxidised silver (C); high vacuum (V).
Typical use was the recovery of optical sound track from cinematographic film during projection and infra red light beam burglar alarms. The type 'C' cathode is unsuitable for exposure measurement with panchromatic films but could be used with orthochromatic (bromide) paper.
Data and graphs from Mullard Ltd., "Valves for Industry and Communications, Part One", p. 449 ff., 3rd. 1952 edition.