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 Cossor label is stuck to the envelope.
The twin anode sections are blackened to radiate heat more efficiently.
The dome also has the Cossor brand etched into it. Note also the BVA logo.
The oxide coated ribbon filaments are supported into the pinch at the lower end.
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.