The X66 is a selected X65 and also known as CV1099. It is a triode hexode mixer or frequency changer.
The CV1099 also appears on the envelope.
The Marconi paper label.
The top cap connects to the control grid of the hexode. Grids two and four (screens) are strapped together above the top mica. Grid three is connected internally to the grid of the triode that is mounted above the pinch.
The classic envelope is 38 mm in diameter, and excluding the base pins is 96 mm tall.
References: Data-sheet & 1043. Type X66 was first introduced in 1940.