The 6CG3 is an American 525 line colour television efficiency diode and part of the GE Compactron range on the 12 pin base. The 6.3 Volt heater is for parallel connected valves.
As an efficiency diode it is designed for large inverse peak voltages and to return energy back to the HT rail at the end of the working line stroke. Normal AC mains rectification was not part of the design.
See also the 19CG3 that differs only in heater voltage and was designed for series heater chain use.
This end of the anode shows the inner surfaces bent out and the metal appears to be copper plating on a nickel sheet.
This image shows the cathode tube passing into the lower mica and emerging from the upper mica. The white coated helix that can be seen at the top of the valve connects to pin 6 that is not used externally. The heater is a single hairpin. The white helix looks to be extra insulation between the heater and the cathode so as to increase the maximum permitted heater - cathode voltage.
The anode is stitched together from a pair of pressed sheets.
This end of the anode is not opened out to form additional radiating surfaces.
The wide glass tube envelope is 28 mm in diameter, and excluding the B12C base pins is 70 mm tall.
Reference: Datasheet. Type 6CG3 was first introduced in 1967.