The ME920 is a magic eye tuning indicator, from 1937, designed for universal mains set operation. The AC/ME was functionally identical but was designed for AC mains use with a 4.0 Volt heater.
The text panel from the side of the box describes the valve and its use. The 200 mA heater is for series heater chain operation.
The end window is of standard design, and the display will go through all of its variations for a g1 change of 19 volts.
The magic eye was not a British invention. (British sets at first used neon devices as tuning indicators. See3184) When magic eyes hit Britain from abroad in the later 1930s they were already on modern 8-pin bases. Mazda, in a 'great leap backwards', not only redesigned their version to fit an already-obsolescent type of 7-pin base cap but succeeded in redesigning the electrode structure so that the glowing green target burned out after a few months. This defect was partly overcome in later production versions but whereas Mazda ME920s and AC/ME's are common enough, those which still glow bright enough to be seen in normal indoor lighting are as precious as gold dust.
The classic envelope is 35 mm in diameter and, excluding the B7 base pins, is 83 mm tall.
References: Data-sheet & 1040. Type ME920 was first introduced in 1937.