Omega’s Speedmaster Mark series – with the 1983 Omega Speedmaster Professional "Mark 4.5" Ref. 176.0012 automatic chronograph here being an honorary member – was Omega’s attempt to follow the immense success of its Omega Speedmaster Professional line of Moon watches, in particular following the its 1969 presence on the Moon and the bragging rights that followed (and continue to this day). 
 
Omega decided to come up with a successor to fit design trends of the 1970s, giving birth to the Omega Speedmaster Professional Mark line, beginning with the Mark II.  Although the Mark 4.5 here is not officially one of the Speedmaster Mark series, it received its nickname from the Omega Speedy community given the 4.5's combination of the Mark IV with the automatic movement of the Mark V.
 
The Mark II came with the same Lemania-based movement as the original Speedmaster Professional "Moonwatch," reference 145.022.  The Mark series contained Omega’s original calibre 861 movement and was in production from 1968 through 1996, when Omega introduced the calibre 1861 movement.
 
Unlike the Speedmaster Professional, the Mark series featured barrel-shaped cases, polished chamfers, and a brushed top and sides, giving it a rather different look from the standard Speedmaster Professional case worn by NASA astronauts.  The classic Speedmaster tachymeter is Professional is known for is also different – instead of residing in an external bezel, it is under the mineral glass crystal, protecting the tachymeter from scratches or disruption.
 
Oh, and let's not forget the legendary badass Charles Bronson wore a Mark - the colorful Mark II racing dial variant - in several films, as well, most notably the 1975 action film, "Breakout."  The film plot - starring Bronson, Randy Quaid, Robert DuVall, and Jill Ireland - revolves around a bush pilot (Bronson) getting hired for fifty thousand dollars to go to Mexico to free an innocent prisoner (DuVall).  The film is notable for giving the usually serious Bronson a more comedic, lighthearted role.
 
Now for some 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." 
 
James Ragan, the NASA engineer responsible for the qualification tests, has spoken about the importance of the Speedmaster by saying, “The watch was a backup. If the astronauts lost the capability of talking to the ground, or the capability of their digital timers on the lunar surface, then the only thing they had to rely on was the Omega watch they had on their wrist.  It needed to be there for them if they had a problem.”
 
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).  Of note, Omega is currently designing a Speedmaster capable of accompanying man in a mission, planned for 2030, to Mars where temperatures range from -133°C to 27°C.
 
This Omega Speedmaster Mark 4.5 comes on an Omega-signed stainless-steel bracelet, and with nylon strap, leather-backed canvas strap, springbar tool, and rugged travel case.