Omega Speedmaster Professional Moonwatch with reference 3872.50.31. The watch is from  1999 and its in very good condition. This watch comes with original box & papers.

  The Omega Speedmaster Professional Moonwatch with reference 3872.50.31 is a discontinued model highly regarded for its "Hesalite Sandwich" configuration. It combines the vintage aesthetic of a Hesalite front crystal with the modern appeal of a sapphire crystal exhibition caseback.

• Movement: Hand-wound Calibre 1863, a higher-grade, rhodium-plated version of the famous 1861. It features a 48-hour power reserve.
• Crystal: Hesalite (Plexiglass) front crystal, which provides a warmer, more authentic vintage look and won't shatter like sapphire.
• Caseback: Transparent (Sapphire) see-through back, allowing a clear view of the intricately finished manual-winding movement.
• Dimensions: Classic 42 mm stainless steel case with a 20 mm lug width.
• Strap: Originally presented on a black alligator leather strap with a stainless steel foldover deployment clasp.
• Collector's Note
  This reference was produced approximately between 1996 and 2004/2006. It is the leather-strap version of the 3572.50.00. Enthusiasts often seek this model because it offers the "best of both worlds"—the original NASA-specified Hesalite front with the ability to admire the movement through the back, a combination no longer available in the current standard Moonwatch lineup.