The 6K8G is the classic triode hexode frequency changer or mixer for broadcast applications. The exhibit has a clear glass bulb and would rely on external shielding where other mixers had external zinc paint sprayed on to provide screening. For isolation of the signal input and output circuits the hexode input grid is taken to the top cap.
The base from pinch to bottom mica. Note that the heater/cathode assembly is displaced to one side this allows for the triode grid and injector grid to be a single structure. The triode anode is the flat sheet seen in the image.
Another view of the bottom mica and electrode layout. The original design for the 6K8 was as a metal valve and electrode structure is shown in that exhibit.
The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 98 mm tall.
References: Datasheet & 1040. Type 6K8G was first introduced in 1938.