The U193 is a television efficiency diode for use in line timebase output circuits, where it would function to return power to the reservoir capacitor during line fly-back. This exhibit, also from Mazda, shows that it is a direct equivalent of the PY801 and an improved version of the PY800.
Typically it will have a heater cathode insulation that is good to 6 kV and a peak current capability of 500 mA.
The reverse of the envelope shows that it was made in England.
In detail the internal connection to pin 3 goes to the anode.
Pin one connects through the anode to a helix of insulated wire that runs round the heater inside the cathode tube.
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 71 mm tall.
Reference: Datasheet. Type U193/PY801 was first introduced in 1960.