The 5902 is a special quality EL71 sub-miniature audio output valve. However the glass carries three designations: 5902, CV4029 and EL71. It is possible that the red paint on the top of the valve indicates the SQ rating.
The Mullard data-sheet says the 5902 is for use where high ambient temperatures, shock and vibration are part of the operating environment. The 5902 has statistically controlled major electrical characteristics.
This exhibit was the driver for a pair of 2E26 modulator valves in the Plessey PTR175 aircraft transceiver. See the 2E26 page for pictures of the modulator.
The Mullard name and inside can be seen the suppressor grid.
The data-sheet indicates that this valve should be given extra cooling and this tube was the mounting for this 5902.
The inside of the heat-sink tube with a corrugated liner.
The thin glass tube envelope is 10 mm in diameter and, excluding the base wires, is 42 mm tall.
Reference: Data-sheet. Type 5902 was first introduced in 1959.