The CY1 is a Ct8 based valve and is identical electrically to the CY31 which is on an IO base. Philips owned Mullard and as this is marked Made in England it is likely to have been made on a Mullard production line.
This half wave rectifier was designed to operate on universal supplies and it has a 200 mA heater. The maximum reservoir capacitor is 32 µF and the minimum series resistance to limit in-rush current is 125 Ohms.
The CY1 and CY31 were introduced in 1939 as replacements for the UR1.
This exhibit employs a mesh anode.
The mesh anode in close-up. The operating anode surface is a metal tube with the outer mesh acting as heat-sink.
The classic envelope is 42 mm in diameter and, excluding the Ct8 base pins, is 96 mm tall.
References: Datasheet & 1043. Type CY1 was first introduced in 1939.