The UBL21 is one of very few double diode audio output pentode valves that were designed. This exhibit has lost all marking but comparison with EBL21 gave a positive identification. The heater only drew 40 mA at 6.3 Volts. At 50 volts it took 100 mA - exactly the requirement of the UBL21.
The valve has a common cathode and the diodes are for signal detection and AGC rectification. A large signal was required for good output and sets using these valves were designed for local signal reception in a domestic setting.
The screen below the bottom mica.
The diode anodes are small plates placed either side of the base of the cathode.
A closer view of the diode anodes.
The wide glass tube envelope is 28 mm in diameter and, excluding the B8B base pins, is 77 mm tall.
References: Data-sheet, 1040 & 3002. Type UBL21 was first introduced in 1941.