The 6D6 is a variable μ pentode for AGC controlled amplifiers at RF and IF in superhet radio receivers.
Electrically the 6D6 (UX6 base) is identical to the Type 58 (UX6 base) and the 6U7G (IO base).
The main electrodes are obscured but the pinch construction is visible.
The wire grids of this pentode appear above the internal coating. The control grid is taken to the top cap and to aid heat transfer this grid is wound on copper support rods. The dark cylinder seems to be a screen, it is not a heat sink for the control grid.
The classic envelope is 38 mm in diameter, and excluding the UX6 base pins is 107 mm tall.
References: Datasheet, 1040 & 1043. Type 6D6 was first introduced in 1936.