Omega’s Speedmaster Ref. 3510.50 automatic – like this recently serviced full set example here from 1999, with matching serial numbers – has a lot going for it.  It has an ever so slightly small diameter that the Professional, a clean and striking dial, an automatic chronograph movement, and – importantly – quite a price difference compared to the Professional.  
 
But first a little on the legendary Speedmaster pedigree.  Omega introduced its Speedmaster line of chronographs in 1957, when it was introduced as a sports and racing chronograph, complementing Omega's position as the official timekeeper for the Olympic Games.  Many different chronograph movements marketed under the Speedmaster name.  
 
The manual winding Speedmaster Professional or "Moonwatch" is the best-known and longest-produced; it was worn during the first American spacewalk as part of NASA's Gemini 4 mission and the first watch worn by an astronaut walking on the Moon during the Apollo 11 mission.  The Professional remains one of several watches qualified by NASA for spaceflight and is still the only one so qualified for EVA.  The Speedmaster line included other models, including analog-digital and automatic mechanical watches.
 
Beginning in 1962, NASA – anticipating the need for astronauts on space missions to move about in space outside the ship, thus necessitating the need for a wristwatch to withstand the difficult conditions of space – anonymously purchased a series of chronographs from different watch brands in an effort to find the best watch available for astronauts to wear in space.
 
In 1964, the watches satisfying all pre-requirements were officially purchased by NASA and subjected to a series of tests and pre-selection processes called the “Qualification Test Procedures.”  Only three watches of six chronographs successfully survived this arduous pre-selection phase.  The three remaining were then subjected to 11 different tests – some of the most rigorous trials endured in the history of horology.
 
By early March 1965, testing was complete, leaving – you guessed it – only the Speedmaster.  At the time, NASA’s testers wrote, "Operational and environmental tests of the three selected chronographs have been completed; and, as a result of the test, Omega chronographs have been calibrated and issued to three members of the Gemini Titan III crews." 
 
On 20 July 1969, the first manned lunar landing resulted in NASA astronaut Neil Armstrong as the first to step onto the Moon’s surface.  As the electronic timing system on the Lunar Module was malfunctioning, Armstrong left his watch aboard as a reliable backup.  Nearly 20 minutes after his first step, he was joined by Buzz Aldrin wearing his Omega Speedmaster Professional – making the Speedmaster the first watch worn on the Moon.  Alas, a few months after this mission, Buzz's watch was stolen and never returned.
 
Regardless, it is from here the Speedmaster steadily gained a reputation of a reliable and historic timepiece – since the epic first steps on the Moon, the Speedmaster Professional remains the only NASA-qualified watch for extravehicular activity (EVA). 
 
Now, onto the Reduced.  The Speedmaster Reduced was first introduced in 1988 as a smaller, more affordable version of the Omega Speedmaster; Omega subsequently halted manufacture in 2009.  Starting with a base movement of the Omega calibre 3220, with a Dubois Depraz chronograph, with rhodium‑plated finish and a 40 hour power researve when new . 
 
This configuration provides a combined thickness of only 6.5mm, far slimmer than comparative chronographs of the same calibre.  Following its discontinue in 2009, Omega has yet to introduce a direct replacement, despite the success of the first generation Ref. 3510.50.  Worn & Wound has written an excellent overview of Omega's Ref. 3510.50, here.
 
This Speedmaster Ref. 3510.50 comes with its original bracelet, nylon strap, springbar tool, and Pelican travel case.