The PenB4 is an audio output valve designed for UK all AC sets. With a large output capability this would have been fitted to larger more expensive receivers.
The glass envelope is smoked all the way round.
It is a true pentode with three wire grids. The suppressor grid has a very open pitch. The mesh is the anode and the two struts between g3 and the anode appear to be supports that hold the mica to the top of the envelope. The oxide coated cathode is wide to offer considerable emission. 8.8 Watts for a single ended stage is considerable.
A closer view of the cathode.
The classic envelope is 44 mm in diameter, and excluding the B7 base pins is 115 mm tall.
References: Datasheet, 1040 & 1043. Type PenB4 was first introduced in 1937.