I have one I got from Kevin Kisseberth but no longer need (85). After all I installed your 84 version in my 83 and got another 82 (perfect fit) from Supradad as a future spare or possible restore to original. The 85 has some rust but seems pretty usable. Let me know if interested $35 plus shipping to TX which should come out to around $50 total.
As I saw when checking for the '83 match, all the filler necks from '79 through to the '83 were the same.
The '85 / early '86 mk2 filler necks were solid and attached to the tank with the ring of screws & rubber gasket.
The design of the tube shows a solid accordion shape form at the tank side.
The earlier necks have a rubber hose and 2 clamps connecting neck to tank.
It looks like it's designed to be a saftey thing to prevent it pulling loose and spilling fuel in the event of an impact.
I didn't look at my '84 so I'm not sure what design that year used.
Going from the tank to the cap.. Get it all.
The necks all looked to be the same lenght and should interchange without problems.