The CY1 is a Ct8 based valve and is identical electrically to the CY31 which is on an IO base. 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.
The lettering reads Philips Miniwatt and then the Type designation of CY1G.
Also on the glass is a pair of cap ds separated by an up arrow.
The anode is blackened and made from pressed nickel sheet. However, within the pressed anode is a round tube and the cathode tube sits within that. This construcion appears to be common to all CY1s including the one with the mesh anode.
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.