Mirror Mounting questions..,on previously posted antique dresser.

Still working on the antique dresser I posted previously. Slow and steady. I'm sure this has something to with how the mirror is mounted. There is a hole drilled at the top of the dresser, a smaller one right behind it and the rectangular space that goes through the back and it has a slotted area inside under the top of the dresser. You can see light coming through from the inside It looks like the wood would go through somehow to support the mirror. I am pretty sure the supports for the mirror are not original. : ) The last photo is what the previous owners had on there to support the mirror. It was just drilled into the back of the dresser. They took the mirror off for easier moving before we got there. Does anyone know the correct way these mirrors are supposed to be mounted to the dresser. and what kind of support they originally had. I would like to try and recreate it. Thank you : )