The CY31 is a half wave rectifier on an International Octal Base. The design dates from 1939 and the CY31 was a replacement for the UR1. This design was made with both Ct8 (CY1) and IO bases. Originally made with a shouldered envelope this tubular version looks to be a later (post war) product. The 200 mA heater was used for universal sets without a mains transformer so that the radio would work on DC as well as AC mains. If connected incorrectly the chassis became live - and lethal.
The maximum reservoir capacitor is 32 µF and the minimum series resistance to limit in-rush current is 125 Ohms.
The anode front to back dimension is only 3 mm.
With cooling fins to the side the anode is 18 mm wide.
The wide glass tube envelope is 30 mm in diameter and, excluding the IO base pins, is 81 mm tall.
References: Data-sheet, 1040 & 1043. Type CY31 was first introduced in 1939.