The 6X5GT is a standard HT rectifier for low power AC mains driven equipment. It was of a type made by several manufacturers, in this case Brimar.
The original 6X5 design dates from 1936 but the Brimar type appeared in 1940.
The output of a centre tapped transformer would be applied to the anodes, the rating given is 325-0-325, the common cathode would be the output for the rectified voltage. A effective series resistance of 350 Ohms was required to limit surges and the reservoir capacitor had to be restricted to 16 µF.
The picture shows the two anodes stacked vertically on the common cathode tube. We also have the version with the anodes mounted at right angles to one another. This form allows for a greater radiating area per anode as can be seen on 6X5GT.
The base cap says Brimar and Made in England but the envelope has the RCA octagon etched into the glass. Is it possible that English base caps were sent to the US?
The side view shows the small diameter of the active part of the rectifier, The width is required for heat dissipation.
A closer view and the connection to the cathode tube.
The wide glass tube envelope of the top valve is 29 mm in diameter and excluding the IO base pins the valve is 70 mm tall.
References: Data-sheet, 1040 & 1043. Type 6X5GT was first introduced in 1936.