The 4101E is a directly heated spherical telephone repeater triode on the GPO bayonet base. The old GPO code is POVT31B. The later CV Type is CV1639.
STC in the UK was a subsidiary of the American ITT. The 4 prefix denotes the STC made valve the American Type was the 101E.
Types 4101D, 4101E and 4101G have small differences one from another. The E variant has a filament designed to reduce splutter noise.
The electrodes are supported by the pinch and by a glass rod that runs vertically and is attached to the pinch tube. In this example shock has broken the glass rod at the base and the electrodes are no longer rigid. One consequence is that the oxide coated filament becomes detached from the top tension springs.
The base cap carries the words Standard Telephones and Cables and Made in Great Britain. The construction of all of these early spherical triodes was based on a ladder grid and flat plate anode placed either side of a central filament.
The UV base is a bayonet type and the locating pip guides the valve into the base where it is turned to lock into the clips. On the left can be seen one half of the detached filament.
The balloon envelope is 59 mm in diameter, and excluding the UV base pins is 95 mm tall.
Reference: Data-sheet. Type 4101E was first introduced in 1925.