Some versions of valves were fitted with a ceramic base cap in place of the normal black moulded cap. There were three reasons why this was sometimes done:
Moulded base caps were slightly water-absorbent; in tropical climates and at high altitudes this affected voltage rating.
The dielectric loss in some plastics could be quite high and with HF transmitter valves these losses resulted in RF heating of the plastic. The consequences of this heating include, sub optimal performance and early valve failure.
The self-capacitance of a moulded base varied with temperature and as it dried out. With HF self-oscillator valves this caused a small but unwelcome frequency drift as a set warmed up. By choosing the right type of ceramic it was possible not only to reduce this drift but to partly offset inherent frequency drifts due to other causes.
Many WWII Service valves were fitted with ceramic bases for these reasons. For example the 807.