The 6B8G double diode plus voltage amplifier pentode was designed for detector and first audio stages 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 pentode or beam tetrode such as the 6V6G.
The base cap has the RCA logo and the brand name Radiotron as well as Made in USA.
The pentode is at the top of the envelope and is contained within the screening cylinder. There are mica pieces at the top to hold the electrodes rigid within the glass by fitting tightly to the shouldered top of the envelope. The screen continues below the pentode and merges with the screening box around the two wire anodes of the signal diodes.
The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 95 mm tall.
References: Datasheet & 1040. Type 6B8G was first introduced in 1937.