The CV1288 is an M-OV DET12 as can be seen by the dual identification on the envelope. The pre CV designations were the Navy NT58 and the GPO POVT121A.
The DET12 is a VHF transmitting triode with an anode dissipation of 50 Watts.
Both the control grid and anode are taken to top pins. The length of connecting wire between the grid and the outside world is shorter than if the grid went to a base pin and this is important when higher frequencies are involved.
The anode is graphite and the filament, like the grid, is helical.
The pinch and lower filament connection. The grid is supported from above.
A closer view.
The balloon envelope is 64 mm in diameter, and excluding the B4 base pins is 152 mm tall.