The CV4059 is identical to the M8097, a special quality version of the commercial EAC91. It is a miniature diode triode and the prototype was the CV137.
The diode is a separate valve to the triode and can be operated in a separate circuit.
Mullard describe the EAC91 as intended for service as an oscillator triode for use as a frequency changer (mixer) and the CV4059 is the special quality version. The sample circuit given by Mullard uses the diode as the non linear mixer element and rates the design as suitable for UHF use up to 300 MHz.
The diode is on the left as a separate section and not sharing a common cathode with the triode.
The triode section with copper supports for the grid winding.
The thin glass tube envelope is 18 mm in diameter and excluding the B7G base pins it is 47 mm long.