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The May 17, 1943, raid was immortalised in the 1955 film The Dambusters and items linked to the operation are in high demand. The button on offer at Northamptonshire auction house JP Humbert (19.5/15/12% buyer’s premium) took £39,500 on July 1 against an estimate of £25,000-40,000.

The Bakelite button was pressed by bomb aimer John Fort on board Lancaster bomber ED 906 AJ-J, the fifth aircraft to attack the Möhne Dam, which was piloted by Squadron Leader David Maltby.

Together with other items from the plane, the button was given by Maltby to the prep school where his father Ettrick was headmaster, Hydneye House, which had a small museum.

Auctioneer JP Humbert says: “The vendor, an old boy of the school, has decided it was time to sell the final items from the collection he acquired when the school closed [in the 1950s].”

Also on sale were four large marbles used by bouncing bomb inventor Barnes Wallis in the development of his weapon and used post-war on his lecture tours. Estimated at £12,000-18,000, they sold for £18,500.

An altimeter specially fitted at eye level to bomber ED932 AJ-G, piloted by Guy Gibson, to aid in low-level flying on approach to the doomed Möhne Dam was also on offer guided at £4000-6000 and sold for £6200, together with a cockpit blackout blind from his plane which made £640.

The original mahogany ‘Dann’ sight used by Fort during the Dambusters raid was sold to a private buyer at JP Humbert in January 2015, for £41,500. It came from the same collection.

This curious triangulation instrument – the two arms were placed at an angle to line up with the towers at either end of the dam so that the bomb could be dropped at precisely the right distance from it – was the brainchild of Wing Commander Dann.