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. 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 equivalance between the M8097 and CV4059 is demonstrated by the presence of both type designations on the envelope.
The triode is at the top of the picture with the single diode at the bottom.
A close-up of the triode showing the Copper grid supports and that the cathode coating has been applied to just the length of cathode tube that fits into the grid.
The reverse of the envelope.
The mica support discs can be seen to have their points on the outer edge bent down as the assembly was inserted into the envelope tube. This interferance fit holds every part of the structure rigid.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 46 mm tall.
References: Data-sheet & 1040. Type CV4059 was first introduced in 1956.