This exhibit is a full wave rectifier with a type designation of 5Z4G and GZ30 the construction looks like that of the Brimar 5Z4G, Pinnacle were re-sellers.
Professional equipment, including radar, etc., sometimes required circuits in which valve cathodes carried large signals and had to 'float' at voltages well below that of the chassis. Glass-envelope '-G' versions of common types were therefore required. The original metal envelope 5Z4 had to have the outer envelope at below cathode potential to prevent them acting as an anode so were at a disadvantage in these applications.
The GZ30 is a post war designation whereas the 5Z4G is from the 1930's.
The twin rectifier sections seen end on. The width seen here is the diameter of the central active element, the rest of the anode is for cooling purposes.
An anode front view. The cathode tube with connection and also the heater can be seen above the bottom mica.
The classic envelope is 44 mm in diameter and, excluding the IO base pins, is 100 mm tall.
References: Data-sheet & 1043. Type 5Z4G was first introduced in 1940. See also1940 adverts.