The M8079 is Mullard's special quality version of the EB91 double diode. These diodes are for signal rectification at medium frequencies eg video.
The two diodes are separated by a full height metal plate screen. The anodes are made of two thin plates 10 mm long and 8 mm high, pressed to leave a vertical 2 mm diameter tubular cavity. Into this cavity is fitted the cathode assembly.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 45 mm tall.
References: Data-sheet & 1040. Type M8079 was first introduced in 1963.