The EBC33 is a double diode triode. The design dates from 1938 with the introduction of the Mullard Red series.
Mullard specify the device for use as an AF voltage amplifier for use at the end of an IF chain where the diodes would demodulate the AM signal and rectify an AGC control voltage, and the triode would act as the first audio amplification stage.
The external screening metalisation precludes an inspection of the actual construction of the valve.
The classic envelope is 35 mm in diameter and excluding the IO base pins and central locating spigot it is 88 mm tall.
References: Data-sheet, 1040, 1043 & 3002. Type EBC33 was first introduced in 1938.