The magic eye class of tuning indicator originated in the US and Type 6E5 was one of the earliest types to hit the market. Almost all European types are more-or-less directly copied from it.
Early specimens had short lives due to ‘burning’ of the fluorescent target and poisoning of the cathode coating in the centre of the target area. This was later cured by enclosing this part of the cathode within the spiral of a small space charge grid.
Early (pre-war) specimens lack this extra grid, later replacements have it. Our specimen looks fairly early, and well-used.
The display changes from open to closed for a 7.5 Volt change on the grid.
The end window envelope is 36 mm in diameter and, excluding the UX6 base pins, is 84 mm tall.