The U339 is a television efficiency diode. The function of the device is to enable the stored energy of the line scan coils to be returned to the supply rail during flyback.
The valve is rated at 5,000 Volts (PIV) and a peak current of 600 mA. The valve is unusual in that the top cap is taken to the cathode. This means a 5,000 volt heater cathode insulation.
The anode cavity is a cylinder. The cathode tube is quite large and thus allows for high peak currents.
Seen end on the anode plates have been left bright. In fact this is the inside surface of the anode. The anode is folded from a single carbonised sheet into an H configuration that provides considerable radiating area. The treatment is on one side only of the sheet.
The wide glass tube envelope is 29 mm in diameter and excluding the IO base pins it is 74 mm tall.