The HVR2 is a half wave low current EHT rectifier from Mullard. It would have been designed for provision of EHT in such applications as Radar and Oscilloscopes where Cathode Ray Tube indicators were used.
The anode supports are glass and fused into the pinch.
The white tube is the oxide coated cathode. The wide separation of the cathode from the anode is because its high voltage design and provides extra insulation.
The classic envelope is 45 mm in diameter, and excluding the B4 base pins, is 112 mm tall.
References: Data-sheet & 3002. Type HVR2 was first introduced in 1937. See also1937 adverts.