Type 6J7 was originally designed as a detector, however, 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.
This American made valve carries the old UK military stores code of 110E/105 and was thus made for use in the UK.
Further round the base cap is the crown and AM logo of the Air Ministry. This was a valve for the RAF.
The screening cylinder hides the inner pentode structure.
The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 96 mm tall.
References: Private communication & data-sheet. Type 6J7G was first introduced in 1936.