Thanks for all the searching guys. Yes, suburban/panel rear fender bolts appear to be quite different than the trucks. Which makes sense, given the difference in the way it mounts.

Here's a photo that shows what I believe to be original parts. I don't believe this has been removed. I've also taken a fender off another panel that had these very same bolt/washer combo.

Suburban/panel fender bolts

tclederman: Your link is broken for me. I actually haven't been able to access that website for some time. Am I the only one?