The A2087 is a noise diode for amplifier testing. The above article gives details of the method.
The GEC brand label on this M-OV valve.
The filament is a single V in the centre of the box anode.
The envelope is massive compared to the horizontally mounted diode. The horizontal mounting minimises the lead inductance. The envelope size is probably dictated by the limitations of the valve making line.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 44 mm tall.
Reference: Data-sheet from Rob Coleman. Type A2087 was first introduced in 1959.