Manufactured by the British Thomson-Houston Company, references to this mic are thin on the ground. In the 1942 Engineering Division Training Manual referring to carbon mics, F.C. Brooker says 'the BBC uses two makes, the Reisz and the BT-H types... We will describe the Reisz type only'.
It is not known whether the BT-H was similar to the Marconi-Reisz or not. The only other 'high-quality' carbon mic construction was the Western Electric Company's push-pull double-button mic where two carbon granule buttons were mounted one on each side of a tensioned steel diaphragm and their outputs combined with a transformer; it is possible the BT-H used this principle.
The three colour shots by Kenneth Young (below) show a mic of this type, revealing the BT-H logo on the rear. The centre shot shows the mic with the cover removed.