The GMU2 ia a high voltage half wave gas filled rectifier. The mercury vapour and inert gas is present to speed-up the heating time and lower the on resistance of the valve. The peak current is 5 A but the average current is up to 1.25 Amps.
The Envelope is marked GEC and the short pins and locating pip indicate a bayonet fixing base.
Note that the filament connections are made of substantial wire.
The screening cage is held in place around the glass tube that carries the anode wire. The screen appears to have no connection to the anode. Also the screen has a central hole and that the anode is wider than the hole.
Another view of the screen and anode within.
The pins are fixed into a ceramic disc.
The wide glass tube envelope is 57 mm in diameter, and excluding the base pins is 203 mm tall.