The 6F8G is a medium-μ double triode where each triode has the same characteristics as the 6J5. The primary use is as a Class A voltage amplifier with resistance/capacitance coupling.
The RCA logo.
In this image the two triode sections are seen side by side an angle of about 30 degrees to the camera. It seems that the lower part of the envelope is largely empty
The classic envelope is 38 mm in diameter, and excluding the IO base pins is 98 mm tall.
References: Data-sheet, 1040 & 1043. Type 6F8G was first introduced in 1943. See also1943 adverts.