The lobe on the ball end of the mirror and the gap in the retaining lip on the mount. This is the best I can do, but after you pull the axel bolt from the bottom, soak the seam with a good penetrating lube and wiggle the arm back and forth. Then line up the lobe by lifting as you move it back and forth to find the alignment so that the lobe can pass through the slot in the mount.
I did use stainless bolts and nuts to replace the lag screws. I drilled through the plywood blocks and had the DW hold the bolts while I barely reached them in the generator compartment to get the nuts started. Have not come loose again though.
Vicki and Mike, Aria 3901- The Wined Up Bus