This exhibit is a 1936 tuning indicator type TV4. 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 EM1 has the same characteristics as the TV4 except for the heater.
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.
The picture clearly shows the construction of this valve with its end window and triode like electrode structure.
The end window envelope is 26 mm in diameter, and excluding the Ct8 base pins is 72 mm tall.
References: Data-sheet & 1043. Type TV4 was first introduced in 1936. See also1936 adverts.