The CY1 is a side contact Ct8 based valve. The CY1 is identical electrically to the CY31 which is on an IO base. This half wave rectifier was designed to operate on universal supplies as 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.
The single anode is much wider (17 mm) than deep (3 mm).
The heater is well insulated.
A closer view of the pinch and above.
The classic envelope is 40 mm in diameter and, excluding the Ct8 base pins, is 90 mm tall.
References: Datasheet & 1043. Type CY1 was first introduced in 1939.