The B5 triode is from the mid 1920s and is marked with the BBC stamp. The base is B4 and the filament required 3 Volts. The envelope is opaque an no details are visible above the pinch.
The balloon envelope is 35 mm in diameter and, excluding the B4 base pins, is 83 mm tall.
Reference: 1003. Type B5 was first introduced in 1923.