The 6C8G is a double triode. Each section has a separate cathode and the first valve has its grid taken to the top cap. This will confer greater isolation from the anode circuit than for valve two.
The first triode acted as a first audio amplifier and the second as a phase splitter. Thus the 6C8G could take the output from a detector or record pick-up and provide the out of phase signals required to directly drive a push-pull output stage.
The paper label proclaims that this American valve was distributed by Marconiphone. Lettering on the base in the Sylvania green describes this valve as for replacement purposes only.
Looking into the envelope the two separate triodes can be seen their anodes blacked to form better radiators. The triode on the left has the grid connection taken to the top cap. The top of the picture shows the filament connections it is not possible to determine whether a series or parallel connection to the heaters is used.
The classic envelope is 36 mm in diameter and, excluding the IO base pins, is 96 mm tall.
References: Data-sheet, 4059 & 3002. Type 6C8G was first introduced in 1937.