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 lettering on this exhibit matches that on the September 1958 advert. Our other Mazda U191 has the logo seen in the October 1960 advert.
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 anode ends are bent outwards to allow a greater surface area to fit within the glass cylinder.
The cathode tube is uncoated outside the anode and the circular ridge stops the tube from moving within the lower mica.
The wide glass tube envelope is 29 mm in diameter and excluding the IO base pins it is 74 mm tall.
References: Data-sheet & 1040.