Your picture does not show the three-pronged 'frame' that completes this piece.
The completed assembly fits through the hole in the door panel. A threaded sleeve (black bezel) screws onto the end of the three-pronged 'frame' to secure it to the panel.
Mine was broken (one of the 'three legs' broken - except I still have all of the parts), but I was able to purchase a complete used unit from Midwest-Bayless and swap it for my broken one. My mirror works perfectly now! Check with them - the units come available from time to time,...
I had pics, but deleted them when I got the functioning unit. I can get pics of my old one, if ya need,...!