Well, that's not a fuel line joint. I'm not sure what it does (no 550 in my yard) but it looks like a water drain fitting for the rubber filter box to carb octopus.
Currently hard to find as you know. Hopefully someone here can help, there have to be some at bike junkyards as well but I doubt if most would know what it is (particularly since the Honda parts fiche just describes it as "piece, air cleaner").
Try searching for the part number 17359-374-000 - this brings up a few listings that might have one. My experience is that most web parts suppliers list everything, and when someone orders something they try and get it - so unavailable parts remain unavailable. Some actually have parts though.