The PM5X general purpose and audio output valve is part of the 1927 six volt range from Mullard. The details on the valve emphasize that there is practically no glow from the filament. The PM filament was an oxide coated azide type.
The alignment of the bulb to base is not perfect and the adhesive tape is probably there as a repair of failed adhesive. Also no paper label exists on this sample. The Mullard logo is etched into the side of the envelope.
The 5X and BVA logo on the opposite side of the envelope.
Looking up into the bulb reveals the horizontal box anode and the grid can be seen extending beyond the anode on the left. This angle also shows the etching in better detail.
The balloon envelope is 45 mm in diameter and, excluding the B4 base pins, is 92 mm tall.
References: 1931 datasheet & 1043. Type PM5X was first introduced in 1927.