The miniature valve is still constructed with end micas and Mullard had a special assembly section dedicated to the hand assembly of these tiny devices and the design dates from 1950.
The grids can be seen through the envelope. The anode looks to be two active plates on either side of the cathode with straps at the edges where the grids are supported and little active electron flow is needed.
The thin glass tube envelope is 10 mm in diameter, and excluding the B8D base leads is 37 mm tall.
Reference: Data-sheet. Type CV466 was first introduced in 1950.