The CV4059 is a special quality version of the CV137 and commercial EAC91. It is a miniature diode triode, it is also identical to the M8097 that is also printed on the envelope.
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). 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 special quality versions were designed for long life and vibration resistance.
The triode on the right with the diode on the left.
The top mica is double thickness and this extra mica largely stops vibration of the supports from enlarging the holes in the mica leading to movement of the electrodes and valve failure.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 47 mm tall.
References: Data-sheet & 1040. Type CV4059 was first introduced in 1956.