The X65 is a triode hexode frequency changer for receiver applications.
Earlier production was encased in the shouldered envelope but here we have a later example in the tubular glass envelope.
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 Marconi brand label.
The row of grid supports matches the image here but the glass in this exhibit makes it almost impossible to see in this image.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 87 mm tall.
References: Data-sheet & 1040. Type X65 was first introduced in 1937. See also1937 adverts.