The ECL86 was one of the last triode plus output pentode valves designed and was released to the market in 1962. Uses were in domestic radio and audio equipment. Normally in single ended output stages for radio-grams and tape recorders. The design proved popular in low cost stereo systems. The ECL86 was preceded by the ECL82.
The construction of this Cossor valve looks exactly the same as the Mullard exhibit and it is likely they both came from the same production line.
The small triode on the left is virtually half of an ECC83. Note the mica support for the top of the triode that sits on a metal screen.
Front view of the triode with pentode behind.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 70 mm tall.
References: Datasheet & 1040. Type ECL86 was first introduced in 1961.