Your AD is correct...
You can switch your solid case back for a sapphire one, but they really keep the solid case back, in this case.
I had this dilemma, as I wanted a Sen ator Navigator with 2 different Case Backs...And it wasn't possible.
I didn't want to switch the solid case back for a sapphire one, even if the sapphire lets you admire the magnificient Cal 100, because it was not historically correct:
Did you ever see a Pilot Watch with a sapphire case back???
So, I was on to buy a noram Senator Navigator, till I saw ( Here ) the LE for Rimowa.
With this LE, you have a Senator Navigator with a very clean dial, and ...2 different case backs!
Here, a pic of the packaging, where you can see the solid case back...
Here the sapphire case back:
Here the dial:
The Rimowa was a LE of 100 pieces for the Asian Market only, and all sold out, as I tried to get one ofr one of my friends, and I was told that it was no longer available...
It will be a rare and difficult bird to catch, my friend.
Hope that these infos will be of some help, though.
Best.
Nicolas