The daughter of a Dambusters crew member has offered a £5,000 reward for information leading to the return of his stolen log book.
The artefact was loaned to military historian Alexander Bateman by the family of Sgt John Fraser for the purposes of research.
He was jailed for two years in February for theft after failing to return it.
Sgt Fraser's daughter Shere Lowe believes someone other than Bateman knows of its whereabouts.
Ms Lowe, who is visiting Lincolnshire from Canada to mark the 74th anniversary of the raids, said: "Some people say it's just a document , a piece of paper, but that's not what it is to me - it's priceless."
"Because I lost him at a very young age - I value every little piece - so to get it back would be like getting a piece of my father back."
She said she did not think it right to treat the log book as a commodity but had been advised that "money talks".
"I believe someone out there knows something, and hopefully they will come forward and tell us where it is," she added.
Dambusters log book theft: £5,000 reward offered - BBC News