The 6V6 family of beam tetrodes came in many constructions and were very popular as the final audio stage of broadcast receivers.
This exhibit was supplied to the UK Govenment as it carries the old stores code of 10E/349.
Through the smoked glass part of the construction can be seen. The control grid is wound on copper rods and has a heat radiating fin above the top mica.
The classic envelope is 45 mm in diameter, and excluding the IO base pins is 99 mm tall.
References: Datasheet, 1040 & 1043. Type 6V6G was first introduced in 1937.