The 5Z4GY is a full wave rectifier. 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.
Designed to supply equipment of up to 40 Watts total power, this rectifier was made by several manufacturers. This suggests a very popular type, and one that found use in military equipment during WWII.
The holes in the anode plates are seen to pass pass right through as can be seen face on.
This last picture shows the pinch on the left with the getter holding ring. The mica supports and the small spikes that form an interference fit to the envelope are clearly visible. The use of these spikes allows small variations in the envelope size to be accommodated and still form a tight fit.
The classic envelope is 45 mm in diameter and, excluding the IO base pins, is 100 mm tall.
References: Data-sheet & 1043. Type 5Z4GY was first introduced in 1940.