The Bristol 'Auditorium True-Tone' was first introduced on the market in 1925 by Pountney & Co. and the regular appearance of these was plain slip-cast black earthenware using the Alkalite material. This substance claimed to enhance the sound of any audible-wave frequancy travelling through the horn.
This example may have been made to order or as a full-frequency range demonstration, as the body is larger by almost 50 than known examples.

(Radio!, Radio! third edition, J. Hill p. 209, fig. 845)
(The History of the British Vintage Wireless Society 1976-1996, J. Hill p.28, fig. 33)