The 4212D is an English valve made by STC to the parent company's design. The American Type is 212D. The 4212D is a power triode with an anode dissipation of 200 Watts. The anode is made of oxidised nickel and was replaced by the 4212E in the late 1930's.
The ceramic top support with filament tension springs above. Note the ribbed anode.
The lower support: the filament can be seen as a ribbon. The anode is clamped around the wide pinch stem.
A view of the etched lettering, anode and grid.
The wide glass tube envelope is 88 mm in diameter, and excluding the base pins is 305 mm tall.