The PC86 is a purpose designed UHF oscillator triode for television reception. Mullard describe the PC86 as: a UHF frame-grid mixer/oscillator triode.
At UHF a coaxial design would normally be used but by connecting the electrodes to several pins the manufacturers achieve the desired result without the complications, and cost, of a disc seal valve and its mountings.
Lamba sold imported valves and this explains the basic marking on the valve.
The small indented rectangle has at the end the working face of the anode. The rest of the metal is for heat dissipation.
The anode seen end-on.
Thanks to Mike Tonkin for sending the Lamba Valves.
The thin glass tube envelope is 20 mm in diameter and excluding the B9A base pins, is 48 mm long.
References: Data-sheet & 1040. Type PC86 was first introduced in 1962. See also1962 adverts.