The 6SQ7 is a double diode triode sharing a common cathode, designed in the early 1940s specifically for superhet receiver use. The diodes would act as signal detector and AGC detectors, while the triode would be the first stage audio voltage amplifier.
The American maker's name.
The Air Ministry logo and the old stores code of 110E/14
The thin metal tube envelope is 25 mm in diameter and excluding the IO base pins the valve is 50 mm long.
References: Datasheet, 1040 & 1043. Type 6SQ7 was first introduced in 1938.