The 12DE8 is a variable μ amplifier pentode for use as an IF or RF amplifier in car radio applications where the anode is operated directly from the 12 Volt vehicle battery. The design dates to 1957 and also features a separate diode for use as signal demodulator or AGC rectifier.
The Westinghouse brand name. On the left is the diode assembly and the much larger structure on the right is the variable μ pentode.
The diode anode is the bright rectangle in the foreground.
The thin glass tube envelope is 21 mm in diameter, and excluding the base pins is 48 mm tall.
Reference: Datasheet. Type 12DE8 was first introduced in 1957.