I came across this old Chalet "Diver" and I can't figure out how to get the back off??? I didn't want to start prying at it if it's threaded, but it looks like maybe a special removal tool may be needed? In my greenness I'm also trying to identify the crystal (which I believe is acrylic because of the striations). It's a pretty cool watch, but info is hard to come by. Any thoughts???