Ok, here's the spoiler...

It's a three-phase full-wave rectifier, aka...
a diode. (but with 6 legs and a specific purpose)

Like these unassuming things that cost a few cents today.

it's what you get if you start asking what if diodes aren't made of semiconductor silicon 😅
Mercury vapor in a octopus-shaped vacuum tube is the answer! 🐙 🩵

Some beauty shots of Mercury Rectifiers.

They were used to convert AC to DC at very high currents such as to power trams.
The metal liquid->vapor of mercury conducts the current.
They're actively decommissioned bc well... mercury. vapor!
These things have 2kg of mercury inside!

The glow is mainly UV from the electrical arcs through the vapor.

https://en.wikipedia.org/wiki/Mercury-arc_valve

https://www.eevblog.com/forum/chat/help-save-the-last-mercury-arc-rectifier-substation/