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 is the lower unit with the triode mounted above the separating mica.
This exhibit has orange lettering rather than the usual white.
The wire grids of the sections can be glimpsed through the notches and holes in the anode.
Here the top triode can be seen to have a small front to back dimension whereas the heptode has a cylindrical anode.
The thin glass tube envelope is 21 mm in diameter, and excluding the B9A base pins is 59 mm tall.
References: Datasheet, 1040 & 1043. Type 12AH8 was first introduced in 1953.