I'm bumping this thread because, after changing out my turn signal indicators and getting everything buttoned back up and giving myself a big pat on the back for a job well done, I looked down and saw these exact bumpers on the floor of my garage. Somehow my age-addled brain remembered seeing those exact bumpers while reading/commenting in this thread almost a year ago, and I came back to find it again.
I've attached a pic of the bolts the bumpers sit under for a bit more clarity. These bolts/washers tighten up into what seems to be a T-nut or something similar, it floats around a little bit and cinching the bolt completely still leaves a little room (without the bumpers). The result is a rear fender that will rattle just a little bit. Adding the bumpers back in solves the rattle, everything is snug again. A little lube on the bumper was necessary to get them to slip over the t-nuts.
@Grayson Bellingham : thanks for starting this thread way back then! I also noticed that, while my bike DOES have that third bolt you mentioned, it just screws into the plastic of the fender so I don't really think it's doing that much. I bet you'll never miss yours.