(pictures fixed, above)
I am able to remove the cases with a firm push of the button (in the first key position), but I had more trouble mounting the cases on the racks.
They should just slap into place, while locked. The top case does this just fine.
I ended up disassembling the red "latch" piece from the E21 cases and filing down some protruding plastic. I also lubed up the area with a film of grease. Now the work much better. The red latch should slide up and down under spring tension easily. If yours hangs up like mine did, that would make installation more difficult.
Now I can slap both locked cases onto the mounts, and remove them with an easier press of the red button.