The 0Z4 (OZ4) is a gas filled full wave rectifier and emission is produced by ionic heating of the cathode. This cold cathode device has working voltage drop of 24 Volts, less than the drop in a vacuum rectifier, around 50 Volts.
The application was where high efficiency was required as in car radio sets. This type of equipment used a vibrator to interrupt a DC voltage to a transformer and produce an almost square wave to the primary. The HT voltage would be produced at the secondary winding(s), rectifier and then the DC passed to a capacitor input filter.
In operation RF filter circuits would need to be fitted close to the valve base to reduce rectifier noise. The metal envelope should be connected to ground.
The base has only five pins fitted.
The maker's name.
The thin metal tube envelope is 25 mm in diameter, and excluding the IO base pins is 50 mm tall.
Reference: Datasheet & 3002. Type 0Z4 was first introduced in 1935.