The Ever Ready A121D is directly equal to the Mullard IW4-350. To avoid problems with heater-cathode insulation one side of the heater is tied internally to the cathode. This necessitates a separate heater winding on the mains transformer.
The heat radiating flanges on each anode are made into two cylinders.
With some difficulty the A11D can be seen on the gold paint of the dome. The word Ever is much easier to see.
A close-up of the electrodes. The white of the oxide coated cathode can be seen running vertically in the centre cylinder section of the anode.
Another close view of one anode.
The classic envelope is 45 mm in diameter, and excluding the B4 base pins is 100 mm tall.
Reference: 1043. Type A11D was first introduced in 1936.