The 117Z6 is an American valve designed for American mains voltages. In simple designs and/or mains battery sets, this rectifier can operate without a mains transformer to give 120 Volts for battery eliminator service, or 240 Volts when used in a voltage doubler configuration.
The maximum reservoir capacitor is 40 µF and to limit the peak current the valve must be operated with a minimum of 100 Ohms series resistance.
The etched logo around the other side of the envelope looks very much like the RCA logo seen on 117Z6GT and it is most probable that Cossor imported the valve and re-branded it.
The two rectifier sections seen from the side.
The wide glass tube envelope is 28 mm in diameter and, excluding the IO base pins, is 69 mm tall.
Reference: Data-sheet. Type 117Z6GT was first introduced in 1940. See also1940 adverts.