The magic eye class of tuning indicator originated in the US and Type 6E5 was one of the earliest types. This exhibit is the smaller Glass Tubular version of the design and unlike the original is fitted with an octal base.
Seen turned through 180 degrees it can be seen that the central section of the triode anode is oval with the outer sections as flat cooling fins and fixing points.
The target and control electrodes.
The display changes from open to closed for a 7.5 Volt change on the grid.
The end window envelope is 28 mm in diameter and, excluding the IO base pins, is 66 mm tall.
References: Datasheet, 3002. Type 6E5GT was first introduced in 1940.