The NR57 is an audio output valve, in fact a type ML4 from M-OV made for military use in WWII. With a 4.0 Volt heater it was designed to be mains driven, so it is surprising to find the output power is so low. However, the data-sheet says that it was for use as a driver for a larger output valve.
Just visible is the lower mica components. The white insulation of the heater is most easily seen.
Another attempt to get an image of the internal structure.
The classic envelope is 45 mm in diameter and, excluding the B5 base pins, is 99 mm tall.
References: Data-sheet & 1043. Type NR57 was first introduced in 1929. See also1929 adverts.
