Type 6J7 is a contemporary of the 'Magic Three', although originally designed as a detector its linear characteristic and good internal shielding (cf. 6Q7GT) made it a natural choice for audio. It was for two decades the international standard valve for small signal audio amplifier stages.
The lettering on the can carries both the Brimar name (UK) and Westrex (USA). Was this for a Westrex sound product or was this an American valve sold in the UK by STC under the Brimar label.
The thin metal tube envelope is 25 mm in diameter, and excluding the IO base pins, is 63 mm tall.
References: Datasheet, 1040 & 1043. Type 6J7 was first introduced in 1936.