The KY50 / U25 is a dedicated television EHT rectifier for picture tube supplies. The designation U25 was the standard and so Mullard added the KY50 in smaller type.
The valve is rated at 19 kV peak inverse volts, and this suggests an output voltage suitable for a 12 inch black and white tube of say 16 kV.
The valve is also rated for sine wave use, and in this case an input voltage of 9,500 volts would be the maximum. The reservoir capacitor would need a value of 0.001 µF or less. In practice the capacitor would be formed between the internal and external graphite coatings on the picture tube itself.
The anode bell: the coated cathode and insulated heater are easy to see.
In this image the rough cathode coating can be seen, as well as how much of the cathode is plain where the lead is welded on.
The thin glass tube envelope is 18 mm in diameter and is 55 mm in length not including the length of the wires.