The UU5, introduced during 1935 is in Mazda parlance is an 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 as is the Tungsram APV4 variant.
The UU4 (350V) and UU5 (500V) were used in larger radios from the mid-1930s onward and remained in valve equivalent lists until the 1960s. They were reliable workhorses and many other manufacturers made their own equivalent - just look at the title sequence and you will see possibly the largest range quoted for any valve on this website!
Osram had an equivalent with initially, the U14 and the U12 which were the same valve intended to meet all HT requirements up to a maximum of 120 mA at 500V, using a capacitor-input filter but as some valves performed better than others on final factory test the best ones were segregated and marked U14 whilst the lesser performers were marked U12. The U14 will tolerate a maximum reservoir capacitor of 32 µF and to have an effective series resistance of 100 Ohms. The filament is a thin inverted V ribbon in each of the two anode cavities.
The 442BU is Cossor's iteration and has a pseudo - parallelogram electrode cage just like the yummy 53KU/CV378 and posesses ribbon tape filaments suspended on coil springs - sounds like a description of a Series Landrover doesn't it?
The 1821 is the Philips iteration of a B4 based full wave rectifier but as the UU5 has twice it's rating, the UU5 may replace the 1821 but the converse is not advisable - a weedy offering from Philips indeed!.
The IW4-500 and IW4-350 are Mullard's pop at a B4 based indirectly heated full wave rectifier designed to work with a maximum reservoir capacitor of 16 µF and a corresponding series resistance of 150 Ohms but showing a maximal voltage ratimg of 500V and 350V @ 125 mA respectively so both are effectively an MU12/UU5 or a R2/UU3 respectively. For both valves, the cathode is connected to one side of the heater. For a marginally higher rating as per Osram's U14, the Mullard DW4 series sufficed.
Any of these devices make a great main rectifier for PX4 and other vintage valve amplifiers using a B4 valve line up and you can have great fun choosing and tube rolling based on the foibles of these valve ranges as described above. 'Never, in the history of thermionic devices has so much fun been made available through so much choice to a most fortunate few.' Did Winston really say that.......?