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.
edit: went out and removed it and took a closer look, and there's a really fine groove that appears like it could take a clip, but its so small a groove I never noticed it before. the clip that fits it must be very fine wire gauge, (and very weak).
The diagram shows parts 1, 2, and 3, on my bike I seem to only have part 2 and its been that way for 28 years. There is another round piece that's in the anti-dive plunger, and never comes out. I suspect it might be part 3 in the diagram, but I'm not sure. The fit between my part 2 and the I.D. of the plunger is a little loose, so that could mean that part 3 is missing if its a pretty thin gauge collar. Mine seems to be a little out of round too, so I suspect I've been riding it all along without the part 3 collar and the hole got hogged out a little bit. Doesn't seem to affect riding or braking though, whatever that's worth.
If anyone reads this and can measure the diameter of parts 2 and 3 in the diagram with a micrometer/caliper and report back, I'd appreciate it. My part 2 diameter measures 0.321/0.322" or about 8.16mm. Also, if you could check the hole in the anti-dive plunger and see if it has an insert that's captive or not I'd be curious about that too.