The 7Y4 is an American design. The Marconi equivalent is the U149. Here M-OV with their Marconi brand have labelled the valve both U149 and 7Y4.
The 7Y4 is a full wave rectifier.
The maximum reservoir capacitor is 40 µF and the minimum series resistance, for current limiting, is not given in the literature.
The anodes are mounted at 90° to each other and by inverting one of them they share the common oxide coated cathode and both have ample full length cooling fins. Note the locating pip on the metal skirt of the B8B base.
The anode plates are carbonised and stitched together.
The dome carries the lettering 495 and 2D4.
The wide glass tube envelope is 28 mm in diameter and, excluding the B8B base pins, is 56 mm tall.
References: Datasheet & 1040. Type 7Y4 was first introduced in 1938.