On August 6, 1945, the atomic bomb codenamed "Little Boy" was dropped on Hiroshima by the B-29 flying Superfortress Enola Gay - named after the pilot's mother.
It was the first atomic bomb to be used as a weapon by the United States Army Air Forces, and effectively ended Japan's involvement in World War Two.
Behind Little Boy was a $23bn (in today's money) top secret weapons program known as the Manhattan Project, led by the United States with the aid of Canada and the United Kingdom.
It was perhaps the greatest secret project of the 20th century, whose roots lay in fears since the 1930s that Nazi Germany was investigating a nuclear weapons program of its own.
To commemorate their work in producing three successful nuclear explosions in 1945, many souvenirs were taken by - or presented to - the 13,000 consultants and scientists who worked on the project.
Today, one of these souvenirs has emerged on the collectors' market: a 160mm diameter Ball race from the construction of the 'Little Boy' bomb.
A spare part from the construction of the bomb, the single, micro engineered Ball race is one of a pair acquired from the collection of the late Professor Samuel Eilenberg, Professor of Mathematics at Columbia University in WW2.
According to experts, the Ball race - 64 years old and one of old two-of-its-kind- was central to the construction of 'Little Boy'.
The Ball race is now on the market and available to collectors, priced £15,000.
Later, just three days after the Enola Gay's mission, the US would detonate the "Fat Man" bomb over Nagasaki - the second of two atomic bombs ever used as weapons.
"Fat Man" was detonated at an altitude of about 1,800 ft (or 550m) over the city, by the B-29 bomber Bockscar.
The warhead had actually been destined for the city of Kokura but the crew found the city obscured by clouds and smoke from a previous bombing.
The 'Fat Man' bomb ultimately ended World War II - being only the second, and thankfully, final, atomic bomb to be used in warfare.
Bockscar is now on permanent display at the National Museum of the United States Air Force, Dayton, Ohio.
Meanwhile, a signed photograph of the whole Bockscar crew signed by aircraft commander Captain Frederick C Bock and his fellow airmen is available to collectors, priced £950 ($1,570).
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