Not alll have a set screw
I know on my 92 the tab simply snaps into place. You may notice a small metal tab slightly protruding from the tab. You will need to pry it out. Odds are it'* locked in prety good.
This type of mounting was pretty short-lived in that the way the tin clip was attached to the mirror is that pins were peened over. Over time, they loosened up a bt and the mirror would be very wobbly. Translate to can see ANYTHING behind you when going down a gravel road.
Anyways, snap the tab free, glue it on, and you will be able to snap the mirror back into place. No tools required.
Hope it helps and I'm glad that they no longer make'em like that.