The BTH B11 is an audio output valve and the design dates from 1927. The filament is thoriated Tungsten.
The etched images on the dome of the envelope identify this valve and the BBC logo places the manufacture of this valve to 1927 as the practice of using the logo ceased by 1928. The filament voltage is 6.0 Volts.
Seen from below the electrodes can be identified.
The balloon envelope is 51 mm in diameter and, excluding the B4 base pins, is 98 mm tall.
As I write the details of this exhibit the time is 07:30 on 01/07/2006 exactly 90 years after the start of the battle of the Somme. The French hard valve was developed in 1916.
Reference: 1043. Type B11 was first introduced in 1927.