The 6B8 American metal valve is a double diode pentode. Such valves were made for the signal detector, AGC detector and first audio amplifier stages of superhet receivers usually as a double diode triode. This valve with a vari-μ pentode offered the set designer the option of using this as final IF stage plus detectors. For local reception a pentagrid converter (heptode), this valve and sensitive output valve may have been all that was required apart from a rectifier.
The 12C8 is electrically identical but has a 12 Volt heater.
The Radiotron brand and Made in USA.
The RCA logo.
The top cap was much more complicated to build into a metal envelope.
The thin metal tube envelope is 25 mm in diameter and, excluding the IO base pins, is 61 mm tall.
References: Data-sheet, 1040 & 1043. Type 6B8 was first introduced in 1937.