The 6J5G is a single medium μ triode is housed within a glass envelope. This is a classic triode design with the popular audio double triode the 6SN7 being a pair of 6J5s in a single envelope.
The electrode assembly is hidden by the silvering from the magnesium gettering.
We also have the Brimar version 6J5G, the Cossor version 6J5G, the later glass tubular version 6J5GT and the American metal version 6J5 within the museum.
The Raytheon name and Made in USA. At the top the triode anode can be seen - quite small within the shouldered envelope.
A closer view of the top of the anode. The control grid has radiating fins at the top. The anode is blackened for better heat radiation and ribbed for strength. A blackened anode radiating towards a mirror surface seems a bit contradictory.
The classic envelope is 38 mm in maximum diameter and excluding the IO base pins the valve is 85 mm long.
References: Data-sheet, 1040 & 1043. Type 6J5G was first introduced in 1936.