The 811A is a power triode for RF or class B modulator service. These valve found much use in amateur transmitters as well as the original commercial apparatus.
A pair as a modulator could provide a peak maximum power of 340 Watts but would usually be run below maximum.
A pair operating in Class C as an RF amplifier at HF (up to 30 MHz) would yield around 130 Watts of output for a DC input of 225 Watts.
The 'A' variant of the 811 has these extra heat radiating fins added to both sides of the anode.
The missing carbonising on the central part of the anode shows that this valve has been run very hot at some time. It also demonstrates why the additional cooling is always added to the centre of the working face of the anode - where the maximum heat is generated.
The top ceramic insulator holds the filament tension springs as well as being a top support for the electrode supports.
The classic envelope is 60 mm in diameter and, excluding the UX4 base pins, is 145 mm tall.
Reference: Datasheet. Type 811A was first introduced in 1954.