THE HAGUE (AFP).- A cunningly disguised stolen work by Dutch contemporary artist Jan Schoonhoven managed to fool experts at the world's largest art broker Sotheby's, who auctioned it in late June for nearly $300,000.

Now Dutch police are investigating and questions are being asked how the modern art relief, sculpted in 1969 by one of the country's best-known fine artists, managed to slip through the net of a carefully managed system to end up on a London auction block.

Simply named "R69-32" -- referring to the year of creation and an identification number -- the white rectangular work which consists of blocks filled with triangles made of papier-mache, disappeared from the Van Bommel Van Dam Museum in the eastern city of Venlo in March.

It was one of four pieces stolen at the time. How they disappeared and who took them remains a mystery and forms part of the police investigation.

Paul van Rosmalen, a Schoonhoven expert who works for Amsterdam's Borzo Modern and Contemporary Art gallery, was the first to spot the deception.

"The work was simply turned on its side and the series number slightly altered," Van Rosmalen told AFP.

He said the thief changed the series number from "32" to "39" therefore passing it off as a different work.

Schoonhoven, who died in 1994, produced numerous works in series from papier mache which were almost identical in appearance and very hard to tell apart.

It was a simple but devious scheme -- and effective enough to fool even the experts at Sotheby's, Van Rosmalen said.

Immediately after the March 22 break-in, Van Bommel Van Dam museum officials notified the London-based Art Loss Register (ALR), the world's largest database of stolen art, about the theft.

Apart from R69-32, three other works, two by Schoonhoven and one by Czech contemporary artist Tomas Rajlich, also disappeared.

Barely three months later however, the now altered "R69-39" showed up for auction at Sotheby's, which was then alerted by the ALR, saying it might potentially match a stolen art work in their database.

Sotheby's, however, said it "confirmed to the Art Loss Register that the title on the back of our work did not match the title in their records."

"The ALR did not follow up to notify us that they nevertheless believed the work was indeed a stolen work," the auction house said in a statement.

At a resulting auction on June 27, "R69-39" fetched a price of 182,500 pounds (213,000 euros, $285,000) and was combinedly bought by the Borzo in Amsterdam and London's Mayor Gallery.

Borzo's Van Rosmalen became suspicious looking at pictures of the work sent by colleagues in London.

"Looking at the pictures I was sent it is very clear that the title on the "2" on the back had been changed to a "9", therefore R69-32 became R69-39," he said.

Sotheby's confirmed that the sale has now been put on ice.

ALR chairman Julian Radcliffe told AFP it did not go further than the initial alert it gave Sotheby's because "neither we nor Sotheby's knew that the number had been changed."

Nobody in the case doubted Sotheby's good intentions either.

As for the three other stolen artworks, just over a week ago, a man walked into an Amsterdam police station bearing the two other Schoonhoven reliefs and the Rajlich work stolen together with "R69-32".

Identified only as "Ryan L." in the Dutch media, the man was arrested on the spot.

Ryan L. however, said he was simply returning the stolen works which he claimed he had bought for 100 euros, Dutch daily NRC Handelsblad reported.

"All three art works have been authenticated as the ones stolen in March," Dutch police told AFP, confirming an investigation was underway.

In the meantime, Theo Manders, the collector to whom the artworks belonged at the time of the March theft, had been paid out 1.1 million euros in an insurance claim, Van Bommel Van Dam Museum spokesman Paul Hermans told AFP.


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