The X65 is a triode hexode frequency changer for receiver applications. The X65 is the commercial version of the RAF pre-WWII VR99. The X66 is a selected X65 and also given the CV number of CV1099.
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 in via the top cap, thus maintaining the greatest separation between grid and anode circuits.
The Osram paper label. The other side has the etched M-OV lozenge with the Type designation within.
The in-line tops of the circular grids. G2 and g4 are bridged together. There is a fifth wire grid that has an open helical form just like a suppressor grid but the only electrode not seen is the triode anode.
The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 99 mm tall.
References: Datasheet & 1040. Type X65 was first introduced in 1937.