The Rank VS39 is clearly a photocell. The screw top-cap suggests an early 1930's date but whereas valves moved to the 'top hat' top cap in the mid 1930's photocells did not necessarily do the same.
The paper label has provision for the maximum operating voltage but in this example it has been left blank.
The thin wire mesh of the anode connects to the top cap and the semi-circular photo-cathode connects to the green wire at the base. No base cap was provided in this design.
The screw top cap connector - missing the insulated cap.
The cathode connection with soft resin filling the space around the wire.
The wide glass tube envelope is 41 mm in diameter and, excluding the base wire, is 120 mm tall.
Updated June 14, 2021.