A420 detector triode was made by Triotron in Austria.
The filament is complete and the measured current at four Volts (0.08 A) helped identify the valve Type. The actual lettering on the glass has deteriorated beyond legibility.
The Made in Austria sticker and within the bright anode with the grid just visible.
The Silver gettering restricts the angles at which the electrodes can be clearly seen. The layout of the supports suggests a single V filament.
The balloon envelope is 45 mm in diameter and, excluding the B4 base pins, is 96 mm tall.
Reference: 1043. Type A420 was first introduced in 1931. See also1931 adverts.
