The GXU1 is a half wave Xenon filled rectifier. The envelope is marked with CV1835, GXU1 and 3B28 and this valve can be used when any of the others have been specified.
The maximum Peak Inverse Voltage is rated at 5,000 Volts. Although the average rectified current is 500 mA the peak value can be as high as 2 Amps.
The GEC logo added as a paper sticker. GEC being the professional brand of the Marconi-Osram Valve company - M-OV.
At the base the two filament supports can be seen but the filament is hidden by the metal work.
The anode end has a re-entrant top seal and the anode connection is held in a glass tube and not directly fixed to the envelope glass. The 1956 Marconi data-sheet shows the outline of the valve with the anode top cap fixed to the top of the envelope. This re-entrant style was a later construction method.
The wide glass tube envelope is 51 mm in diameter and, excluding the UX4 base pins, is 139 mm tall.
Reference: Datasheet. Type GXU1 was first introduced in 1956.