The EM34 is a dual shadow magic eye tuning indicator.
Mullard describe this device as suited to both receiver tuning indication and test equipment null indications. The grid voltage change is 0 - 5 volts and 0 - 16 volts. The display is viewed end on.
The top of the envelope showing the bowl of the screen and the oxide coated cathode.
The magnificent picture above of a working EM34 was donated by Achim van Duren.
The top of the box reveals a lot about the times the valve was made. The price is printed on the box, this was a time of resale price maintenance, that is the maker stipulated the selling price. Today this would be seen as anti-competitive and protectionist and thus illegal. The thirteen shillings price was expensive for the average buyer.
The wide glass envelope is 26 mm in diameter and, excluding the IO base pins is 72 mm tall.
References: Datasheet, 1040 & 3002. Type EM34 was first introduced in 1946.