The 6AK8 has three separate electrode sections within the envelope. The triode and two diodes share a common cathode and one diode has a separate cathode.
The 6AK8 was specifically designed for use in the detector stage of early FM and FM/AM radios. The triode section providing the first audio amplification, one half of the double diode for AM detection and the single diode and one of the diode pair for FM demodulation, probably in a ratio detector circuit. SeeFM Broadcasting and Signal Detection.
The box seen here and the middle box are both shields. This end tower houses the single diode. The anode can be seen through the wall of the screen and connects to both vertical flat bars. The central tower houses the double diode with each vertical bar being an anode.
The triode section is on the right, the single diode is on the left and the middle box is in fact two L section anodes that forms the double diode.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins, is 58 mm tall.
References: Data-sheet, & 1040. Type 6AK8 was first introduced in 1952. See also1952 adverts.
