Type 6J7 dates from the mid 1930's, although originally designed as a detector its linear characteristic and good internal shielding 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 here we have the Mullard version of this American valve.
The back of the valve, no details revealed.
The top cap connects to the control grid and the other grid supports can be seen above the top mica. The outer shell is a screen connected to pin one.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 69 mm tall.
References: Private correspondence. Type 6J7GT was first introduced in 1936.