The DU10 is a directly heated half wave rectifier from Mullard that was introduced in 1929. The valve has only three pins. The base pattern is the British four pin B4 and the control grid pin (pin 2) is omitted.
The envelope markings are etched. The rear shows the requirement for a four Volt filament supply.
The base: the anode pin is marked with an A.
The balloon envelope is 51 mm in diameter, and excluding the B4 base pins is 115 mm tall.
References: Datasheet & 1004. Type DU10 was first introduced in 1929.