GENEVA (Reuters) - A package of diaries said to have been posted to the United States from Britain in the 1960s could provide a vital clue to the origin of a controversial portrait presented in Geneva last month as Leonardo da Vinci's original "Mona Lisa."

But in a twist typical of the intrigue-prone world of art, the diaries -- notes by early 20th century British connoisseur and collector Hugh Blaker -- disappeared and the Washington address they were sent to seems never to have existed.

"Those papers could well provide the key to pushing back the provenance of this version of the 'Mona Lisa' by at least 150 years," Robert Meyrick, an academic and expert on the largely forgotten Blaker, told Reuters.

And, of course, to helping establish if the so-called "Isleworth" variant of the world's most famous painting in the Paris Louvre could indeed be an earlier -- and priceless -- portrayal by Leonardo of the enigmatic, smiling lady.

Blaker, an unsuccessful painter who as a museum curator and dealer had a reputation for recognizing lost Old Masters, found and bought the "younger Mona Lisa" in 1913 -- in, he later said, a nobleman's country house in Somerset in western England.

Sure it was a real Leonardo, he kept it at his home in the London suburb of Isleworth -- giving it its informal identity tag -- until it passed to his sister Jane on his death in 1936.

But Blaker told no one the name of the country house or of the seller. Meyrick, who was invited to the Geneva presentation to talk about the bachelor connoisseur, is keen to solve that mystery for a biography he plans to write.

"I think he must have put the details in his diaries," he said in an e-mail message this month from Aberystwyth University in Wales where he is Head of the School of Art.

"But the very brief published extracts we have give no clue. If we have that knowledge, we should be able to trace how it came into the Somerset family's possession, and where."

GRAND TOUR PURCHASE?

Meyrick theorizes that it could have been picked up by an 18th century member of the family during one of the Grand Tours across Europe undertaken by young English nobles. Many great works of European art came to Britain that way.

After Jane Blaker died in 1947, the painting was eventually purchased by an international art dealer and then lay for nearly 40 years in Swiss bank vaults until last month's Geneva presentation by a Zurich-based "Mona Lisa Foundation."

At that session, Italian Leonardo specialist Alessandro Vezzosi praised its quality but held back from endorsing the foundation's claim that it is the work of Leonardo, who died in 1519. For that, much more work was needed, said Vezzosi.

Some experts who were not present like British professor Martin Kemp of Oxford University, scoffed at it as a poor copy -- although, as foundation member Stanley Feldman noted, Kemp had never actually seen the portrait.

"The controversy underlines the importance of the diaries," says Meyrick.

With other papers and an unpublished novel, they passed after the death of Blaker -- whose keen eye had brought the scorned Italian artist Amadeo Modigliani to the British art public in the 1920s -- to his painter friend Murray Urquhart.

Before he died in 1972 Urquhart, whose son Brian was a key figure in the United Nations in the 1970s and 80s, said he had sent Blaker's notes from before 1931 to a researcher named Charles Woods who had written asking to see them.

According to Urquhart's account, he posted them to Woods at 116 1/2 (Eds: correct) Maryland Drive, Washington DC -- but heard nothing more. "All I can establish is that there is no such address, and probably never was," says Meyrick.

There is also no trace of Woods.

But in 2010, in response to a standing appeal on his website (www.robertmeyrick.co.uk), Meyrick was sent Blaker's diaries for the last five years of his life by a family who found them years before in a junk shop in Gravesend, east of London.

"Did all the papers end up as junk, or did the earlier diaries really go to Washington?" asks Meyrick, who has written widely on British 20th century art.

"Perhaps we will never know, but I plan to keep looking."