by the looks of the structure, I'd say it is a <i forget the name>
it is sensitive to light, sort of like a photo-diode, but in tube format..
there is no heater, and the plate has some sort of mildy radioactive substance on it (like cesium? i forget)
for use of transmitting a signal via visible light, like the audio track on a old film.
I think I've got a similar tube from tim..