"These are the most outstanding Second World War RAF medals I have had the pleasure to handle," said auctioneer Richard Bromell.
"Flight Lieutenant Richard Dacre Trevor-Roper was Gunnery Leader, flying as Rear Gunner in Guy Gibson's lead Lancaster ED932 AJ-G and was awarded 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.
News of the sale, which will be held on March 19-20, came just a week after Antiques Trade Gazette reported on a remarkable Dambusters item being offered on the market for the first time at J.P. Humbert Auctioneers of Towcester on January 20: the original mahogany 'Dann' sight used during the raid on the Möhne Dam.
This is also the first time the Trevor-Roper medal group and related ephemera have been offered for sale.
For many decades, the medals, two of Trevor-Roper's RAF side caps, assorted letters, photographs and even an original script for the 1955 film The Dam Busters with accompanying 'world premiere' programme, have been kept by family members in a box file.
Richard Trevor-Roper's recommendation for the DFC stated: "This officer flew as Rear Gunner in the leading Lancaster which attacked the Möhne Dam. During the run in, his aircraft was subjected to stiff opposition from flack defences situated along the top of the dam.
"By using his rear guns with great skill, even though bullets were passing through the rear structure of the aircraft just behind him, he succeeded not only in deterring the aim of the defences but later in drawing the fire away from other aircraft which were actually making their bombing runs.
"As Gunnery Leader, Flight Lieutenant Trevor-Roper has set a magnificent example which is an inspiration to all the other gunners in the squadron."
On the day of his investiture he was photographed alongside Barnes Wallis.
Aged 28, Richard Trevor-Roper was killed in action a year later when his Lancaster was shot down over Nuremberg on March 30-31, 1944 during what became the costliest RAF Bomber Command mission of the war. He is buried at Durnbach War Cemetery.
Also included in the lot are the Great War medals awarded to his father, Major Charles Cadwaladr Trevor-Roper, who served with The Hampshire Regiment and like his son was killed in action. His uncle, Geoffrey Trevor-Roper, was also killed in the First World War.
The collection will be offered as a single lot with an estimate of £40,000-60,000.
Dambusters groups are rare on the market. More than a decade ago, medals belonging to RAF flight engineer Ivan Whittaker, the recipient of two DFCs including one for his part in the Möhne Dam operation, were sold at Spink in 2004 for £12,000.
The bomb sight 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.
It is thought to be the only surviving example of those used during the raid and is estimated at £20,000-25,000 at J.P. Humbert, who 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.