The UU5 ia another classic Mazda indirectly-heated HT rectifier. One side of the heater is connected to the cathode. 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 work-horses. Used in large numbers by the British services, there are plenty of good ones still around.
The twin anodes seen side on. The central rod connects to the cathode tubes of both rectifier sections.
The identification is etched into the dome.
The classic envelope is 45 mm in diameter, and excluding the B4 base pins is 102 mm tall.
References: Datasheet, 1040 & 1043. Type UU5 was first introduced in 1936.