I've puzzled this out, finally! As a reference for others, here is how you assemble these mirrors and install them on a "classic 911" (mine is 1987).
Assembly order is very important!
1. Get the mirror housing and mirror base painted. You can't paint the parts properly once assembled.
2. Install the inner motor frame with the three self tapping screws (provided in the kit) into the mirror housing:
3. Mount the swivel plate (plastic cover on the bottom of the mirror housing) with a single small rivet (my factory kit did not include these):
