A previous owner has scraped away some of the external metalising to reveal the triode within.
The anode has an oval shape to the working face.
The heater is a flat corrugated ribbon that runs vertically through the oxide coated cathode tube. The vertical rod in the foreground looks to be for the top connection of the heater.
The dome carries the Type designation, the makers name and the words Barium Tube. Barium tube refers to the Azide process but this valve looks to have a spray coated cathode and the Azide process was not suitable for indirectly heated valves. Thus Barium tube is used as a brand identifier.
The classic envelope is 37 mm in diameter, and excluding the B5 base pins is 96 mm tall.
Reference: 1043. Type R2018 was first introduced in 1932.