The 931A is a photomultiplier, but was not always used as such.
This exhibit was used as a noise source in a jamming transmitter system. Obviously used to generate a spectrum of white noise.
Operating conditions: an anode voltage of 100 V and a cathode current of 0.025 mA. The gain is 1,000,000 and the sensitivity is 10A/Lumen at a wavelength of 420 nM.
The photomultiplier is a cold cathode device.
The wide glass tube envelope is 28 mm in diameter and, excluding the special base pins, is 72 mm tall.
Reference: Datasheet & private communication. Type 931A was first introduced in 1950.