BBC D/11 amplifier, c.1943. The D/11 amplifier was produced on a large scale as a high-grade general-purpose amplifier. This particular unit came from BBC Londonderry. The full BBC number on it is D/11/656. This is a rack-mount unit, and has a very rusty face plate.
The four types of amplifier used in the radio broadcast chain were referred to by the BBC as A, B, C, and D amplifiers. The first 'D' amplifiers, with a gain of about 45dB, were often used to amplify incoming Post Office lines.