It's not that early, ZA.21270 is going to be mid-WW2.
A quick rummage in the trusty Wireless for the Warrior Volume 2 appendices reveals: Amplifier Loudspeaking No.3, C/S No.2 and the note that it's identical to the No.10 apart from having a different cord, and is a 'transverse current carbon microphone'.
So some kind of P.A. application rather than radio.
Chris.
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