The EMM801 is a double bar style tuning indicator in a standard 21 mm glass tube. The EMM801 was used as a recording level indicator in stereo tape recorders, and also found use in tuners etc. where an indication of the relative magnitude of a pair of signals was required. The EMM801 is not a common valve.
The main picture shows the rear of the envelope and the picture above shows the valve turned slightly. The metal skirt is 58 mm in diameter.
Looking at the screen of this used valve it can be seen that the position of each illuminated bar is visible due to screen burning.
This image shows the electrode assembly for one half and is seen edge on. The mica obscures the details.
The rear of the electrodes seen in close-up. The controlling grids are clearly visible. These grids run horizontally across the whole width of the envelope. The construction is more involved than just having two thin indicators mounted side by side.
The thin glass tube envelope is 21 mm in diameter, and excluding the B9A base pins is 55 mm tall.
Reference: Datasheet. Type EMM801 was first introduced in 1957.