The 25Z6GT is a pair of rectifiers in a single envelope with a common heater connection. This construction offers a flexibility in circuit design. The 25Z6GT can be used as a half wave rectifier with both sections in parallel. A full wave rectifier by joining the cathodes or as a voltage doubler.
The Haltron version has pinch construction and is 8 mm taller.
Electrically the 25Z6GT is identical to the 25Z6 and 25Z5.
The twin anode sections. The heater looks to be passing through the cathode tube nine times and the heaters of the two rectifier sections are connected in series by a tape at the top.
The RCA logo, trade name and Made in USA.
The wide glass tube envelope is 29 mm in diameter, and excluding the IO base pins is 63 mm tall.
References: Datasheet & 1040. Type 25Z6GT was first introduced in 1938.