I know this post is 2 years old, but Rob, you are a lifesaver.
Your detailed description facilitated the repair of a wing mirror in Istanbul, after it was snapped off last night. Thankfully whoever did it, left the mirror. 2000 miles back home without a driver's side mirror does not appeal. I have a spare mirror at home in the UK, but that's no good here.
Thank you very much.
Other comments, the white plastic half moon piece was mostly broken into pieces when removing the rusted bolt from inside. As mentioned by other posters. But this doesn't seem to matter, as long as the ring section at the topmost part of this plastic piece is intact, a good sturdy washer in place of the bottom half seems to suffice.