Being more of a rock enthusiast than an art lover at the time, the young doctor put it aside and promptly forget about it for over half a century.

Stumbling across the box of drawings again during a move in 2016, he decided to give them to the Tajan auction house to have them valued.

The head of Tajan's Old Masters department, Thaddee Prate, quickly identified the hand of a master, without specifying who, and valued it at between 20,000 and 30,000 euros.

If Jean B. thought he was lucky then, he was about to become far more fortunate. 

Another expert, Patrick de Bayser, concluded that the two-sided drawing – the other side is inscribed with scientific studies of candlelight  - was by none other than Leonardo, an opinion backed by a third specialist, Carmen Bambach of the Metropolitan Museum of Art in New York.

The attribution of the St Sebastian drawing to Leonardo is absolutely solid.

"The attribution of the St Sebastian drawing to Leonardo is absolutely solid," she told The New York Times, noting that it was complementary to other drawings of St Sebastian by the artist.

A 'national treasure' 

In an instant, the work's estimated value rocketed, reaching between 8 and 12 million euros.

But it's not every day that a new Leonardo turns up and the French government soon stepped in, designating the drawing a "national treasure" and giving itself 30 months to acquire it on behalf of the Louvre museum in Paris, home of the Mona Lisa.

An offer from the state duly followed – 10  million euros – but with a new valuation estimating the drawing at 15 million euros, Jean B. refused and the culture ministry threw in the towel.

But the affair did not end there. 

Jean B. promptly applied for an export permit to be able to sell the drawing to a foreign buyer but the culture ministry refused, claiming that the drawing may in fact have been stolen.

On Wednesday, the retired doctor will ask a Paris court to order Culture Minister Roselyne Bachelot and a senior official in charge of art collections to allow the drawing to leave France.

After the 'Salvator Mundi'

Jean B.'s lawyer Olivier Baratelli termed the government's handling of the painting's discovery as  "catastrophic".

"A culture ministry worthy of its name would have ensured the French state acquire such a drawing," he argued.

The controversial 'Salvator Mundi', purported to be the work of Leonardo da Vinci. Photo: Francois Guillot/AFP

Meanwhile Jean B. has also fallen out with Tajan.

Saying he learned through the press of their plans to put the drawing up for auction without his consent, he revoked their sale mandate.

Baratelli claimed that Tajan had been "intoxicated" by the prospect of a hefty commission after the sale of Salvator Mundi attributed to Leonardo which set a record for the most expensive painting ever sold at auction in 2017 when it was acquired by the Saudi royal family for $450 million at Christie's.

Tajan, for its part, has sued the drawing's owner for 2 million euros for breach of contract, asking that he compensate the auction house for all the work it has carried out on his behalf.