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Appearing on the market for the first time is the original mahogany 'Dann' sight used during the famous 'Dambusters' raid on the Möhne dam on May 17, 1943.

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 brain child of Wing Commander Dann. It was employed by bomb aimer John Fort on board the AJ-J, the fifth aircraft on the mission piloted by David Maltby.

The 'bouncing bomb' guided by this sight was the one that successfully breached the Möhne dam.

Along with other items, it was given to Ettrick Maltby, David's father, after the raid, for display at the museum in Hydneye House, a prep school he owned and ran. When the school was sold in the mid 1950s, it passed to the new headmaster and thence to the current owner, a former pupil.

Inset with a brass plaque reading AJ-J Op Chastise, Möhne Dam, May 17, 1943, it has been verified by George 'Johnny' Johnson, the last surviving member of the Dambusters raid. The bomb sight is thought to be the only surviving example of those used during the raid and is estimated at £20,000-25,000.

"Spine-Tingling"

J.P. Humbert's are also offering the map light and parallelogram used by Vivian Nicholson, Maltby's navigator, on the same Lancaster bomber, together with four of the marbles used by Dr Barnes Wallis during the designing of the 'bouncing bomb' and Guy Gibson's leather collar box. All with the same provenance, they will be offered as separate lots.

Auctioneer Jonathan Humbert said: "Of all the exciting items that have come under my hammer over the years, these have to rank as some of the most spine-tingling and historical. An integral part of modern British history and synonymous with heroism of the highest order, we are very proud to offer these items for sale and, given their excellent provenance, we are hopeful of a great result."

The sale takes place from 5pm on Tuesday, January 20.