The 25Z4G half wave rectifier was designed for series heater use. This Tungsram implementation is built as a full wave rectifier with the heaters in series and individual connections to the cathodes. The joining of the cathode leads must be within the base shell.
The maximum reservoir capacitor specified is 16 µF and the minimum effective series resistance has to be 100 Ohms. The sensibly equivalent U31 is rated for 120 mA and a 32 μF reservoir capacitor - quite different.
The two sections seen side-on.
The two rectifier sections. The central support rod also is the fixing point for the strut that connects the two heaters together.
The classic envelope is 38 mm in diameter, and excluding the IO base pins, is 86 mm tall.
References: Datasheet & 1040. Type 25Z4G was first introduced in 1939.