In the olden days , when they wanted to show off the movements used in their timepieces, they had to make it a see-through caseback in order to appeal to the buyers, in this case the Royal Italian Navy.
But once approved for production, the actual products did not of course have a sapphire back.
By the way, here is a comparison of the caseback of the original Egiziano 2/56 and the re-edition.
Original:

Re-edition PAM 341:

Hope you enjoyed it!
Cheers mate
Stephen