The blue arrow points to the mirror control lever, and the green arrow points to its catch lever. The catch lever is what moves out of the way when it’s not supposed to, and I’ll need to remove the mirror box to fix it.

Mirror Catch at Ready by P F McFarland, on Flickr
In this photo, the catch has disappeared behind the main sequencer, allowing the mirror control to move past. The magenta arrow points to the Kick Lever for the mirror, and the yellow arrow points to the dog on the Main Drive Gear that actuates the kick lever. That part is working fine.

Mirror Catch at Fault by P F McFarland, on Flickr
More photos at
https://www.flickr.com/gp/7699588@N07/178i6V
PF