The 6F8G is a medium-μ double triode. Primary use is as a Class A voltage amplifier with resistance/capacitance coupling. A decade later the ECC82 was providing almost identical characteristics in a miniature envelope.
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 thin glass tube envelope is mm in diameter, and excluding the B9A base pins is mm tall.
References: Datasheet, 1040 & 1043. Type 6F8G was first introduced in 1943.