The ARDD1 is a double diode The AR stands for (UK) Army Receiving.
The visible cylinder is a screen strapped internally to the cathode. Within the screen are two small anodes mounted vertically around the cathode.
The direct equivalence between the ARDD1 and the commercial 10D1 is made clear by the etched lettering on the envelope.
At the top of the electrodes, and inside the screen is the first anode it is a bright plate between the lower support rod and the upper rod closest to the cathode. The anode forms a cylindrical shape around the central indirectly heated cathode. The second anode cannot be seen.
The classic envelope is 38 mm in diameter and, excluding the B5 base pins, is 91 mm tall.
References: Datasheet & 1043. Type ARDD1 was first introduced in 1936.