The TT11 is a small low power VHF transmitting beam tetrode used in WWII.
The Marconi paper label.
The anode is of blackened for improved dissipation by radiation and oval in shape. The cathode and grids are flattened. Surrounding the grids and spaced away from the anode is a bright metal box cage with open sides. This is the beam forming plate assembly.
The classic envelope is 32 mm in diameter and, excluding the IO base pins, is 87 mm tall.
References: 3002. Type TT11 was first introduced in 1947.