The A39A is a tuning indicator Mullard type TV4 made for Ever Ready. 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 design 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.
The Voltage amplifier triode below the display bowl.
The bowl and beam guides connected to the triode anode.
The end window envelope is 26 mm in diameter, and excluding the Ct8 base pins is 70 mm tall.
References: Data-sheet & 1043. Type A39A was first introduced in 1936.