The 12AH8 is a triode heptode frequency changer. The triode oscillator couples internally to the heptode grid number three. The design is aimed at HF communications receivers where oscillator stability is important.
The triode oscillator runs with 100 Volts on the anode to generate a 9.4 Volt LO signal. The heater is centre tapped so that either a 12.6 Volt supply across pins 4 and 5 can be used or a 6.3 Volt supply across pins 4 & 5 together and pin 9.
The heptode section is at the bottom with the triode above the middle mica.
The triode grid can be seen at the top of the image.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 60 mm tall.
References: Data-sheet, 1040 & 1043. Type 12AH8 was first introduced in 1953.