The 442BU is a full wave rectifier from Cossor. The data sheet shows a family of these rectifiers. It is probable that the 442 designation was awarded to the device when tested. The New Process Cossor label is stuck to the envelope.
The twin rectifier sections.
The flattened cylinder anode is clamped to the support via a central flange. The filament is an oxide coated ribbon.
The top of the envelope: the filament tension springs sit on a mica sheet fitted above the main top mica. The springs are fully compressed when the valve is cold and will expand upwards as the filament expands at working temperature.
The classic envelope is 50 mm in diameter and, excluding the B4 base pins, is 110 mm tall.
References: Data-sheet & 1043. Type 442BU was first introduced in 1931.