The UU5 ia another classic Mazda indirectly-heated HT rectifier with a characteristic envelope shape. One side of the heater is connected to the cathode. The anode cavities are each made from two black pressed plates. This rectifier is designed to work into a maximum reservoir capacitor of 8 µF.
The UU4 (350V) and UU5 (500V) were used in larger radios from the mid-1930s and remained in the lists until the 1960s. They were staunch workhorses. Used in large numbers by the British services, there are plenty of good ones still around.
The classic envelope is 43 mm in diameter, and excluding the B4 base pins is 100 mm tall.
References: Data-sheet, 1040 & 1043. Type UU5 was first introduced in 1936. See also 1936 adverts.