The 12AD6 is a low voltage heptode self oscillating frequency changer for car radio applications where the HT is the vehicle battery at 12 Volts.The local oscillator grid is g1 and the signal input grid is g3.
The superhet would use this frequency changer, an IF amplifier, detector AGC stage, 12K5 driver and a transistor output stage.
The end view - a featureless anode.
The anode face view, some of the grids can be seen.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 46 mm tall.
References: Data-sheet & 3002. Type 12AD6 was first introduced in 1956. See also1956 adverts.