The U191 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. See also PY301
The side view of the anode. The shaping adds to the area for heat radiation.
The construction is the modern glass disc base as in the all glass valves. The lead out wires are soldered to the octal base pins.
The wide glass tube envelope is 29 mm in diameter and excluding the IO base pins it is 74 mm tall.
References: Datasheet & 1040.