The CV90 is a disc seal triode developed during WWII to operate at frequencies up to 3 GHz. The DET22 was an improved higher performance replacement for the CV90 that was developed during the war.
The CV90 disc seal triode was used in a post WWII type 505 British Naval transmitter.
The small closed cylinder at the bottom is the heater/cathode assembly. The top circular surface is flat and faces the grid wires that can be seen running vertically. The anode is the disc seen behind the grid mesh. The latter disc is the tip of the anode that extends into the large circle of copper.
Small disc seal triodes like this were also used in signal generators.
The re-entrant envelope provides access to a coaxial heater connection. The grid connects to the smaller ring and the anode connects to the wide copper disc.
The top of the anode connection.
The anode descending to the flat disc of the grid with cathode below.
The grid looks to be an early form of frame construction and has been made separately from the grid connection disc. The details can be seen in the full sized image.
The thin glass tube envelope is 20 mm in diameter and, including the base pin, is 84 mm tall.