This is the Sapphire Crystal Steel Case Back Ring Only (NO CRYSTAL INCLUDED) which was fitted to the Speedmaster 3573.50.00 model  and the current model. 
These are Omega supplied service case backs which fit the 42mm Manual Wind Moon Watch 3573.50.00 case reference 145.0811. These service case backs 112st1589 have the same 1589 code as the latest bezel.
You can also install the Limited Edition Crystals I have for sale like the SNOOPY, GEMINI 4 and APOLLO 11 to give your watch that extra special look.
 
I can offer these assembled with either of the crystals mentioned which I recommend as you need special Omega tooling to install the crystals or you can snap them.
 
N.B. THESE ARE THE STEEL RING ONLY, YOU ALSO REQUIRE A 062SN5126 CRYSTAL AND GASKET. IF YOU ARE CONVERTING A STEEL CASE BACK MODEL YOU ALSO NEED TO CHANGE THE MOVEMENT RING AND AN EXTRA RUBBER O-RING.
 
This was fitted to the model stated above but will fit all the other case references for the 42mm Speedmaster Manual Wind Moon Watch models back to the mid 1960's once they introduced the crown guard Speedy.