This exhibit is a late-1930s type TV6, tuning indicator. It has a 6.3 V heater whereas the (otherwise similar) Type TV4 had a 4 V heater. The prototype of the TV6 was the Philips Type EM1. After a year or two the Mullard TV6 was re-designated Type EM1 in accordance with Philips policy of phasing out all British valves in favour of Continental types.
Magic eye tuning indicators of this class originated in America during the mid-1930s in order to assist the correct tuning of superhet receivers. Some early British superhets used neon tuning indicators but the Magic eye type was immediately popular when M-OV started to manufacture copies of RCA types in 1936/7.
The TV4 was the first Mullard magic eye on the UK market. It was more compact than other available types (which made it popular with set designers) but early specimens had short lives. This was corrected by alterations to the internal design. It is possible that the TV6 designation was changed to EM1 in order to bury the poor reputation which the 'TVs' had by then acquired.
The picture clearly shows the construction of this valve with its end window and triode like electrode structure.
The picture shows the back view of the envelope.
The amplifier triode section below the bowl of the screen.
Looking into the target, the deflector electrodes are seen as flat plates. The oxide coated cathode is seen to be devoid of the grid like structure that was added to later designs to prevent short life times.
The end window envelope is 26.5 mm in diameter and, excluding the Ct8 base pins, is 70 mm tall.
References: Datasheet, 1040 & 1043. Type TV6 was first introduced in 1937.