The RCA Receiving Tube manual describes the 78 as being electrically identical to the IO based 6K7 apart from the capacitances, thus this is one of the Magic Three valves.
The Brimar name, lower screen and connections between the electrode assembly and the shaped leads fixed into the pinch.
The Type designation within an Octagon outer leading to the suspicion that this is a rebranded American made valve.
Looking up from the base clearly shows the cylindrical anode within the outer screen.
The classic envelope is 38 mm in diameter, and excluding the UX6 base pins is 96 mm tall.
References: Data-sheet, 1040, 4055 & 1043. Type 78 was first introduced in 1937. See also1937 adverts.
