The VT-126B American Government specification is equivalent to the commercial 6X5GT/G and both designations appear on the valve.
It is a standard HT rectifier for low power AC mains driven equipment and the design was produced by several manufacturers. The original 6X5 design dates from 1936.
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 as well as the etched Type designation 6X5GT/G.
The makers name - National Union - appears on the base. The form of the stacked anodes is clearly seen as they fit around one another.
The wide glass tube envelope is 29 mm in diameter, and excluding the IO base pins is 69 mm tall.