The 6B8G double diode amplifier pentode was designed for use as the detector and first audio stage in radio receivers.
The diodes would be for signal and AGC detection and the high impedance audio pentode would provide good small signal gain for feeding into an output Valve. The pentode is described as semi-vari-μ so that gain can be controlled by the AGC line.
The diodes are below the screened pentode and above the pinch.
The box anode is not split into two and so it looks as if the second diode anode is the pair of vertical rods.
The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 101 mm tall.
References: Datasheet & 1040. Type 6B8G was first introduced in 1937.