The 6P3P is the Chinese Type designation for the 6L6G. It is a larger more powerful audio beam tetrode than the 6V6 receiver type. Modern 21st Century valves are intended only for audio applications.
Looking along the grid axis the additional heat sinking is placed in the centre of the active anode faces.
The control grid wire can be seen as can the beam plates.
The cathode is a rectangular tube and the heater is a pair of hairpins.
The 6P3P on the left is this exhibit. For details of the amplifier click here.
The classic envelope is 51 mm in diameter and, excluding the IO base pins, is 116 mm tall.