A low power triode for use in telephone repeater systems, equivalent to Western Electric 101D
It has a 1 amp directly heated cathode
This valve would have been manufactured for use by the Postmaster General's Department (PMG) for use in their carrier telephone systems.
Manufactured by: Standard Telephones and Cables, England (most probably).

Physical Description

Spherical bulb with exhaust tip on top with black bakelite four pin UV style base

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