The EM1 is a magic eye tuning indicator. The control voltage swing required is 5 Volts to move from minimum to maximum shadow. The later EM4 is a dual control type whereas this has a single control range.
The window is circular with a bowl shaped target.
A second example of the EM1 in poor cosmetic condition. The electrodes forming the display are fully revealed.
Where the paint has flaked away the inner triode amplifier anode can be seen. As a low power voltage amplifier the anode is left bright and is quite small.
The wide glass tube envelope is 25 mm in diameter and, excluding the Ct8 base pins, is 71 mm tall.
References: Data-sheet & 1043. Type EM1 was first introduced in 1937.