The U24 EHT rectifier was designed for use in an RF EHT supply and dates to 1948. Around this time RF oscillator generation of EHT was being developed as referenced in the 1948 Mullard advert.
The anode bell connects to the anode and the bright tube below is a screen.
Here the two supports within the screen can be seen to be the heater and cathode fixings. The upper support goes above the oxide coated cathode and connects to both cathode and heater. The lower end connection is hidden by the screen. The heater looks to be a single strand. Also clearly seen are the side mica sheets holding the anode firmly in place within the glass envelope,
The wide glass tube envelope is 28 mm in diameter, and excluding the IO base pins is 80 mm tall.
Reference: Datasheet. Type U24 was first introduced in 1948.