The 20F2 screened RF pentode was designed for use in universal (i.e. no mains transformer) sets with a series connected heater chain. Mostly in television receivers at RF or as a sync separator.
This exhibit is of the first style of B8B based valves in that it is of all glass construction but also has has a metal skirt with both a locating pip and a long central spigot. Later B8B valves evolved into a locating pip moulded into the glass envelope.
The rectangular cathode tube, copper control grid supports topped with a heat radiating fin.
The solid base pins, spigot and locating pin on the side of the skirt. In some B8A base designs one glass pin support would be moulded into a square to denote the correct orientation of the skirt - but not here.
The thin glass tube envelope is 20 mm in diameter and, excluding the B8A base pins, is 54 mm tall.
References: Data-sheet, 1040 & 3002. Type 20F2 was first introduced in 1949.