I finally got home and pulled my caliper off, and the parts diagram is making more sense to me now.
I took a closer look into the hole in the anti-dive piston rod, and there is in fact some kind of non-metal collar inside there. It looks like hard plastic, perhaps that's what the OP saw mangled in his. Never noticed that under the brake dust before.
Then I remembered back to the first time I had the caliper off and the little metal collar fell on the ground. I put it back in from the left side, thinking that's where it fell from, but it also fits in from the right side as well. Swapped it around to the right side, put the caliper back on, and it went back together without any issues. So I think I've had it reversed all these years without knowing it.
The final mystery, the little retaining clip, appears to have been missing from mine since I first got the bike, I don't recall ever seeing it, and that probably explains why the collar fell on the floor the first time I pulled the caliper. I'm technically the second owner, but bought the bike when it was two months old with 1,000 miles on it, so the caliper should have been original when I first removed it. The end of the collar protrudes a bit from the piston hole, and appears to align into the hole in the caliper bracket where the bolt goes through. So I'm guessing there's enough tolerance in there for the clip to attach to the collar, and the caliper bracket can still slide over the whole thing and into position to accept the bolt. The collar itself has a machined groove in the end that appears to be where the retaining clip would slide over it, and the groove is pretty thin, so the clip must be too.
So to anyone who has looked at these things lately, do you see the retaining clip in place where I'm describing it?