This exhibit is a 1938 tuning indicator type TV4A. It has the then standard British 4.0 Volt heater, whereas the (otherwise similar) Type TV6 had the new American standard 6.3 Volt heater.
The TV4 was the first Mullard magic eye on the UK market in 1936. 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 and operating perameters.
The anode current increased slightly to 0.22 mA and the target voltage was reduced from 250 Volts to 200 Volts. The minimum grid voltage altered from -5 to -18 Volts but the shadow angle changes remained the same.
The TV4A was packaged in the same style of box as the PM22D.
The construction shows the amplifier triode section below the display.
The glass makes the image less than perfect but the insulated heater is seen at the base and the common cathode passes through the top mica as does the triode anode connection.
The display bowl with deflector, cathode and grid.
The end window envelope is 26 mm in diameter, and excluding the Ct8 base pins is 74 mm tall.
References: Datasheet & 1043. Type TV4A was first introduced in 1938.