Interesting use of them valves/tubes

Hey Guys,

I know just enough of valves/tubes to be adequately dangerous, but keen to learn from the specialists here. All in good time.

I saw this in a museum recently. It's a vintage metal-detector with what seems to be a amplifier board, possibly for the amplification/processing of the audible sound the metal-detector made ?

Thought the application of these were just brilliant.