The GR TT 1931 Protos I saw in Geneva came with a ... plexy.
I asked about this point to several people, and I always got this answer.
I tested myself the watch, and I can confirm that it was a plexy.
BUT... There is a big BUT... The definitive watch may well come with a sapphire, as I was told later, hence I preferred to not state anything definitive on this matter.
Of course you're right in the distinction you made between the Deluxe and " Not Deluxe " Vintage Reversos.
Best,
Nicolas.