hey Dean, I'm curious about the part(s) that are shown in the diagram because my bike doesn't seem to have the same set of parts that are shown in the diagram.
On mine, its just a single flange piece, that always falls out when I remove the caliper and I have to remember that the wide side of the flange goes to the right side when re-inserting it.
All it seems to do is reduce the hole in the anti-dive piston to the size of the lower caliper bolt that goes through it. As you say, its captive once you tighten down the caliper bolt, so I don't know what the purpose of the retaining clip would be, unless that's what's supposed to keep it from falling out in the first place when you remove the caliper. I've looked at mine a few times, but never all that closely, and I never noticed a place where a clip would even be used.
Is it possible that this clip existed in earlier model STs, and later models don't actually have it? The parts fiche doesn't look quite like the service manual diagram, so I'm not able to make sense of it clearly there to see what parts are even available.