The 4020A is a telephone repeater triode. The envelope is also marked as a GPO No. 73 POVT73A.
The design dates from 1937 and the directly heated filament is oxide coated. The 4020 came in three variants: the A with the four short pin bayonet base, the B with the B4 base and the C with the bayonet base but the control grid brought out to a top cap.
The 1947 STC 3A/108A was developed from the 4020 design.
The internal structure is mounted at 45 degrees within the envelope. The filament is a single hairpin with the tension spring at the top. The grid is wound on a U shaped support that is held at the top by a central rod.
The data says that the 4020A was introduced in 1937, whereas this is serial number 96 and is dated 10/10/1936.
Here the lower gid winding and supports can be seen. The bright anode has the supports spot welded to the side seems.
The flat face of the anode has a circle embossed on it to add strength.
The balloon envelope is 57 mm in diameter, and excluding the American 4-pin bayonet base pins is 98 mm tall.