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.
This Mullard valve was made by the parent company Philips - probably in Eindhoven.
The original EM1 has a recessed top glass rather than the dome seen here. However, this looks to be a later build of the original design with four shadows.
The reverse is red paint.
The Ct8 base.
The display bowl. The visible deflector vane is a flat strip.
The cathode with guard wires. Two of the flat vanes are clear.
Part of the discolouration of the display phosphor.
The discolouration indicates a four segment display and there are four guide vanes.
The wide glass tube envelope is 26 mm in diameter and, excluding the Ct8 base pins, is 73 mm tall.
References: Data-sheet & 1043. Type EM1 was first introduced in 1937. See also1937 adverts.
