It looks to me like a cable tidy, the sort of thing normally found round the headstock to keep all the wires and cables in check.
You could always try here... https://www.worldoftriumph.com/collections/bikes Find your model and look through the exploded diagrams to see if you can find it
It’s the source of that annoying rattle you’ve been trying to locate. Sensible answer now, it looks very homemade to me though for what purpose I haven’t a clue.
