The U31 half wave rectifier was designed for series heater use.
The maximum reservoir capacitor specified is 32 µF and the minimum effective series resistance has to be 100 Ohms.
The Osram paper label on this M-OV valve.
The etched lozenge is distinctive of M-OV production. The insulated and coiled heater is made-up of two separate units joined in the centre to form a single heater. The cathode can be seen to be connected by a tape to the pinch.
The classic envelope is 45 mm in diameter, and excluding the IO base pins, is 104 mm tall.
References: Datasheet, 1040 & 1043. Type U31 was first introduced in 1937.