Moonwatch is considered a manual wind Speedmaster Calibre 1861 with a hesalite crystal, closest to that with a sapphire display back is the now discontinued 3572.50 which had sapphire back and hesalite front, the new 3573.50 has sapphire both sides, which is nice, but not as close to the original as the 3572.50.

This is a good example of the 3572.50: http://www.ebay.com/itm/OMEGA-Speed...456187580?pt=Wristwatches&hash=item3ce000d0bcPurchases made through these links may earn this site a commission from the eBay Partner Network

 

Its because the original worn by the NASA astronauts had hesalite, and even the current 3570.50 (solid caseback) has hesalite. It has a warmth to it, and reflects light differently to sapphire, it can also be polished up easily, and in the event that it gets too damaged to be re-polished, the replacement is cheap, while a sapphire is very expensive to replace.

 

Well that one on eBay is $2,600, they're all used at this point as they've been out of production for a while

http://www.ebay.com.au/itm/261456187580?clk_rvr_id=625097201982Purchases made through these links may earn this site a commission from the eBay Partner Network

 

I just went with one of these for my vintage Speedy:

http://omegaforums.net/threads/105-002-62-on-horween-shell-cordovan.12333/