The GU5 is a half wave mercury vapour rectifier. The latter exhibit has a clearer glass envelope and so the internal structure is easier to see.
The maximum reservoir capacitor is 4 µF.
When the filament is first energised the inside of the envelope becomes coated with a grey mercury film. As the valve warms up the film evaporates. Applying a few volts to the anode produces a blue glow. The filament did not glow red at full filament volts.
The warning label - allow filament to run for 15 minutes after transit or storage before applying HT. The mercury vapour rectifier can be a delicate beast.
The B4 base pins and the shape of the electrodes.
The coiled tape filament is supported at both ends and in two places in the middle.
The classic envelope is 50 mm in diameter and, excluding the B4 base pins, is 126 mm tall.
References: Datasheet & 1040. Type GU5 was first introduced in 1936.