To satisfy an Illinois State law requiring me to drive with both side outside mirrors, I just installed one on the right side of my '31 Roadster (original stanchions). To be safe, I slipped a thin nut inside the stanchion and threaded the mirror shaft into the nut. The inside shape of the stanchion held the nut securely and kept it from turning.
Unfortunately, the repo mirror is far from original and sticks out (to me) like a sore thumb on my car which has only been maintained and never taken apart.
Gar Williams