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 seen full face.
The anode end on. Between the lower mica discs can be seen a wire helix inside the cathode tube. This looks to be present to ensure that the heavily insulated heater is kept away from the cathode walls.
The wide glass tube envelope is 28 mm in diameter and excluding the IO base pins it is 73 mm tall.