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The raid was immortalised in the 1955 film The Dambusters and items linked to the operation are highly sought-after in salerooms.

This button is on sale at Northamptonshire auction house JP Humbert (soon to be Humbert & Ellis) on July 1, estimated at £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 and 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 is time to sell the final items from the collection he acquired when the school closed [in the 1950s].”

Also on sale are four large marbles used by bouncing bomb inventor Barnes Wallis in the development of his weapon and used post-war on his lecture tours. Estimate £12,000-18,000.

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 is also on offer guided at £4000-6000,together with a cockpit blackout blind from his plane estimated at £400-600.

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Cockpit blackout blind from Dambusters plane estimated at £400-600 at the July 1 JP Humbert auction.

Bombsight bought

The original mahogany ‘Dann’ sight used by Fort during the Dambusters raid was sold to a private buyer at JP Humbert on January 20, 2015, for a hammer price of £41,500 (estimate £20,000-25,000). 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.

Dambusters medal groups

Also in 2015, the medal group of Les Munro, the last surviving Dambusters pilot, was due to be offered at  London saleroom Dix Noonan Webb on March 25 with an estimate of £40,000-50,000, to raise money for the RAF Benevolent Fund's upkeep of the Bomber Command Memorial in London.

Before the sale, Munro accepted Lord Ashcroft's offer to donate £75,000 to the fund in return for gifting his decorations and medals to a museum in the RAF veteran's native New Zealand.

Mr Munro, now 95, was one of only two New Zealanders to have taken part in the raid.

DNW decided to waive all their fees and out-of-pocket expenses on the understanding that the Museum of Transport and Technology in Auckland, where the honours will go on display, makes a further donation of £10,000 to the RAF Benevolent Fund.

However, the British peer failed in his bid to buy another Dambusters medal set also offered in March 2015. Ft Lt Richard Trevor-Roper’s Distinguished Flying Cross (DFC) and Distinguished Flying Medal (DFM) pair, being sold by the family, doubled the top estimate to make a hammer price of £120,000 at Dorset auctioneers Charterhouse.

Ft Lt Trevor-Roper was gunnery leader, flying as rear-gunner in Guy Gibson's lead Lancaster ED932 AJ-G, and won the DFC for his part in the Möhne Dam attack on May 16-17, 1943. He received the DFM (gazetted December 23, 1941) as a sergeant with 50 Squadron.