This exhibit came labelled as an X05 and looking at the main image this seems reasonable. However, close scrutiny with a powerful light shows that it is in fact another X65.
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 into the top cap grid, thus maintaining the greatest separation between grid and anode circuits.
The up arrow indicates that this valve was destined for the RAF.
The top of the envelope shows the hexode mixer section. The triode is below.
The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 96 mm tall.
References: Data-sheet & 1040. Type X65 was first introduced in 1937. See also1937 adverts.