Dr Peter Brueggemann, 59, who lives in Norfolk, spent eight years and £70,000 building the Dreidecker Dr 1 fighter from scratch, but has now sold it for £60,000 to British pilot Richard Vary.
Sir Albert Ball wrote note to the mother of pilot Lothar von Richthofen
He was the younger brother of the famous Red Baron Manfred von Richthofen
Lothar shot down and killed Mr Ball s son, Captain Albert Ball, in May 1917
Sir Albert said in note that he was sending Mrs von Richthofen a photo of his son
He added that they should both be proud to know both sons did their best
Lothar survived the war but was killed in a flying accident in 1922