The 6DC6 is describes as a semi-remote cut-off pentode. A variable μ valve aimed at IF amplification.
The main picture shows the anode either side of the valve with a strap holding them together. The central column has the heater, cathode and two inner grids visible. The grid wires pass flat across the flat cathode. Grid 3 is in fact a pair of beam forming plates.
Here the grids are seen end on. The variation in the spacing of the control grid is small.
The RCA logo with the grids visible through the rectangular hole in the beam plate.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 46 mm tall.
References: Data-sheet, 1040 & 1043. Type 6DC6 was first introduced in 1954.