The UBL21 is a double diode output pentode. It can produce 4.8 Watts of audio at 10% distortion. The literature gives the cathode resistor as 200 Ohms and the load impedance as 3,500 Ohms.
It is probable that this valve was designed to produce final speaker drive direct from the output of the detector. This would suggest use in broadcast receivers that were intended for local station reception at a budget price.
The anode has holes in the side affording a glimpse of the internal grid structure. It is a true pentode with all three grids being visible.
The anode in side elevation.
Thanks to Robert Ganski for sending the .pdf for this exhibit.
The wide glass tube envelope is 28 mm in diameter, and excluding the B8B base pins is 76 mm tall.
References: Data-sheet, 3002 & 1040 Type UBL21 was first introduced in 1941.