The PC86 is a purpose designed UHF oscillator triode for television reception. Mullard describe the PC86 as: a UHF frame-grid mixer/oscillator triode.
The item of note is that the logo is the later Mullard style and the manufacture is described as imported. This is much better than the earlier marking of imported valves as 'foreign'. Foreign was used when we in the UK were asked to believe that British was best and so the term foreign always had the connotation of inferior.
The small anode end view. The width is 5 mm. In the tutorial referenced above is a diagram of the re-entrant anode.
The working faces of the anode are very close to the grid with the heat radiating portions much further away.
This is another example of a similar vintage but carries the older pejorative wording of foreign.
The thin glass tube envelope is 20 mm in diameter and excluding the B9A base pins, is 47 mm long.
References: Data-sheet & 1040. Type PC86 was first introduced in 1962.