That explains it. Cheers.Many cars from the 50s had the rear view mirror mounted this way but by the 60s the were hung up high like they are now.
This mirror was mounted on the dash because the rear air conditioner blocked the view of the standard location of the rearview mirror. The rear air conditioner is suspended from the roof above where the second and third seats back up to each other. It is shown at the 45 second mark of the video when the guy pulls the saddle out of the back.
It was a pretty rare option.