This valve is badged as a Navy receiving valve, an NR83. Unlike most valves supplied for government service this exhibit is also clearly labelled with the commercial Type 6J7G.
The 6J7G is a contemporary of the Magic Three and almost as ubiquitous. Although originally designed as a detector its linear characteristic and good internal shielding (cf. 6Q7G) made it a natural choice for audio.
It was for two decades the international standard valve for audio amplifier input stages and audio systems generally.
A close look under the outer screening fails to reveal any detail.
The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 95 mm tall.