The CV1921 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.
This exhibit is also marked with the Mazda brand and the commercial Type of U24 thus indicating direct equivalence.
The anode cylinder at the top with a screening cylinder below.
The oxide coated cathode tube with heater within. The heater cathode insulation would be small and so one side of the heater is connected to the cathode. One support connects to the base of the cathode. The other passes above the cathode as an inverted J and holds the top of the cathode.
The wide glass tube envelope is 29 mm in diameter, and excluding the IO base pins is 78 mm tall.