The CV1099 is a triode hexode mixer valve (prototype X66), the CV denotes a specification for the UK government. Another close commercial valve is the 6K8G.
The triode was designed for local oscillator use and an injection voltage of 10 V peak to peak was produced. The hexode mixer took the signal into the top cap grid, thus maintaining the greatest separation between grid and anode circuits.
The electrically identical VR99 is in a classic envelope. It is probable that this exhibit is a post WWII valve made for replacement purposes.
As CEI was an American company it probably has rebranded a valve originally made in the UK.
The 6K8G within an octagon is typical of American companies. Maybe just the base cap is UK. 6K8G from General Electric looks identical in construction.
Looking at the electrodes across the valve with the triode anode just inside the screen.
A closer view with a little enhancement.
The electron flow is across the valve with the cathode offset and the triode anode in the foreground. A diagram of the electrode arrangement can be found here.
The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 96 mm tall.
