Type 6J7 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. The Mullard-Philips type EF37A, for many years the standard British low-noise audio pentode, is really a 6J7G under another name.
During WWII Mullard, and others, purchased valves from America to sell as their own. The term "Amerty" identifies these valves.
The 6J7G within an octagon suggests an RCA made valve.
Within the bright cylindrical screen is the dark anode and the three wire grids. The white cathode can be seen at the centre.
The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 99 mm tall.
References: Private communication & data-sheet. Type 6J7G was first introduced in 1936. See also 1936 adverts.