The UCH21 is an early example of all glass construction. This triode heptode frequency changer was designed to operate on a heater chain across the mains supply, and the anode voltage is correspondingly low to take account of the low HT voltage present in universal sets. Mullard describe the valve as designed to accept ACG to the heptode.
The reason why there is no cathode/g5/s pin is that the metal spigot is used for this purpose. This meant that the metal skirt operated at cathode potential, not chassis potential, creating insulation problems unless a suitably-insulated base socket was used. It also creates problems with some commercial valve testers.
The triode sits below the heptode around the common central cathode. It is required to generate a peak to peak voltage of 13 Volts to drive the mixer correctly. The mesh screen obscures the heptode.
The curved sheets abobe the bottom mica are screens, one for the heptode anode and the other for the heptode control grid. The heptode control grid receives the signal and is supported on copper rods.
The top of the glass carries more information including the fact that this was made by the parent company in Holland.
The wide glass tube envelope is 26 mm in diameter and, excluding the B8B base pins, is 60 mm tall.
References: Data-sheet & 3002. Type UCH21 was first introduced in 1941.