Do you know why people love the classic Omega Speedmaster Moonwatch? One of the more reason is that its whole design changed a bit little over the years. No watch can remain in continuous production for over half a century without some capacity involved. Nevertheless, Omega still sells a version of the Speedmaster that is nearly the same to the original design from 1957.
Today, Omega provides two different configurations for their classic, manually wound, Speedmaster Professional Moonwatch. The first is the traditional version that uses a stainless steel caseback and a Hesalite (acrylic/plastic) crystal; the second model swaps out the solid caseback for a display back, and the Hesalite crystal for a sapphire crystal, making a nickname of ‘Sapphire Sandwich’ among enthusiasts and collectors. Even if the movements used inside the two watches are the same from functionality aspect, the version inside the ‘Sapphire Sandwich’ (caliber 1863) is more luxury decoration than the variant in the regular version (caliber 1861), as it is displayed constantly via the watch’s sapphire caseback.
Omega firstly started using display casebacks on the Speedmaster roughly 1980, but they were only fitted to limited edition or numbered pieces, rather than regular production models until around 1987. For a short period of time between 1987 and 2003, Omega manufactured a different, regular production run variation of their classic, Speedmaster Professional Moonwatch, which offered the best of both worlds for customers, and was a 50/50 mix between the two versions of the Speedmaster Professional Moonwatch that can be found in current Omega catalog.
The reference 3592.50 was the first non-limited edition of Speedmaster Professional to receive a sapphire caseback. From the front, the watch looks the same with the regular Speedmaster Professional, complete with its signature, domed Hesalite crystal. While you can see gilt-finished, caliber 863 movement through its sapphire caseback if you flip the watch over. The reference 3592.50 remained in production until about 1996, almost all version left the factory with tritium dials and hands originally due to the timing of its production run.  
Omega replica released the near identical reference 3572.50 to replace the reference 3592.50 around 1996. The main difference between the two watches was the movement that was fitted to them. The reference 3572.50 received Omega’s caliber 1863 movement, with rhodium finishing and Geneva stripes rather than the golden-color of the caliber 863. Roughly 1998, Omega began fitting the reference 3572.50 with dials and hands that used Super-LumiNova to provide them with their luminescence, instead of radioactive tritium like previous version.
By 2003, Omega had discontinued the reference 3572.50, and announced the reference 3573.50 to replaced it, which swapped out the domed acrylic crystal of its predecessor for a box-shaped sapphire crystal, earning it the nickname of the ‘Sapphire Sandwich’ Speedmaster Professional. Actually, the ‘Sapphire Sandwich’ was the same internally to the reference 3572.50 and still featured Omega’s caliber 1863 movement visible through its sapphire crystal display caseback. While, since the different shape and material of its crystal the overall look and feel of the dial-side of the watch was changed obviously.
The reference 3592.50 and the reference 3572.50 are being discontinued, however they now have several different nicknames. During the time that they remained in Omega’s collection, they were simply known as sapphire-back Speedmaster Professional replica watches, while many collectors now regularly refer them to as ‘Hesalite Sandwich’ Speedmasters. Alternatively, a few number of individuals choose to call them ‘Mullet’ Speedmaster Professionals, because of their ‘business in the front’ (NASA-approved Hesalite), ‘party in the back’ (sapphire crystal with decorated movement) construction.
It would be a quite hyperbole that the Speedmaster has remained ‘unchanged’ since its initial version launched more than sixty years ago. Nevertheless the current production model is about as close to the original as it could hope for from a modern watch. While the standard Speedmaster Professional Moonwatch is reliable to the original, and the ‘Sapphire Sandwich’ is an absolutely contemporary timepiece with a classic aesthetic, for the person who not only wants a classic Omega Moonwatch but also to appreciate the complex inner workings of Omega’s most iconic timepiece, the ‘Hesalite Sandwich’ Speedmaster references are the best choice undoubtedly.