Posting as much for a puzzle as anything.
Somewhere around the rear fender.
I think I know where they fell from/where I should put them back but it seemed worth posting as I managed to disassemble without noticing them drop and they are the kind of thing that could easy go missing.
They go underneath the fender. There's a sandwiched setup with washers etc. Mine fell out when I unbolted everything. Took me a few minutes to finally get the correct sandwich orientation. Wish I could remember what it was now. I can't recall if they drop in from the top or come up from the bottom. They go through frame holes.
I think this is correct.
Picture with one in and one out.
There's a metal piece that fits over then the bolts go though.
These are the rear two of the four bolts. The front bolts go in from the top and have nuts and washers from below. No rubbers on them.
It is possible that the rubbers should be under the plastic section shown. (The one that extends back to hold the license plate.)