Special helmet for the F4J-UK Phantoms after they were Anglicised with equipment refits.
It is basically a Mk10, with US oxygen mask receivers installed to the utside of the shell.
when the F4J was brought to the RAF (Ex-US) the aircrew initially flew with American HGU-33 helmets and MBU-14 oxygen masks. The Phantom's British upgrades included the flight equipment, but the British of the time masks would not work with the retained American oxygen system, so a compromise must have been struck where the American MBU-14 mask was installed to a British helmet by adding the external receivers, and it was designated the Mk14.
It should be noted that the aluminium castings of the oxygen mask receivers are special for the Mk14 helmet, as the shell of the Mk14 is more curved at the front openeing than American helmat shells such as the HGU-33.