The 12A6 was designed for use as a single end audio output stage for radio receivers. The Ferranti details give a cathode resistor of 375 Ω and a load resistance of 7,500 Ω. The original design was American.
The base has pin 6 omitted.
The thin metal tube envelope is 25 mm in diameter and, excluding the IO base pins, is 67 mm tall.
References: Data-sheet & 1040 Type 12A6 was first introduced in 1942.