AMSTERDAM, May 16 (Reuters) .

Dutch art dealer Jan Six made the discovery of a lifetime at an auction house in 2016, when he saw the hand of Rembrandt in an unknown painting that had gone unnoticed for four centuries.

The portrait of a well-dressed young man with red hair was presented on Wednesday as the first "new" Rembrandt to surface since 1974. It will be on display at the Hermitage Museum in Amsterdam for a month.

With the help of an unnamed investor, Six snapped up "Portrait of a Young Gentleman", painted around 1634, for a bargain at 137,000 pounds ($185,000) at the London auction. Given past sales, it will likely now be worth very much more.

"Finding a Rembrandt is a tremendous feeling", Six told Reuters.

Six has a special relationship with Rembrandt, having grown up in a house filled with classic Dutch artwork, including a Rembrandt portrait of one of his ancestors, a former Amsterdam mayor, also called Jan Six, as the centrepiece.

With his knowledge of the artist and the period, Six noticed a particular type of collar the subject of the painting wears, which was only in fashion for a short time around 1633 and was painted in a style that only Rembrandt used in those days.

The specialist on Dutch and Flemish old masters then spent 18 months using X-ray techniques and analysis of paint samples to prove he had in fact bought a real Rembrandt.

The 39-year-old art dealer eventually won the backing of more than a dozen Rembrandt experts, including the former leader of the Rembrandt Research Project, who spent a year verifying its authenticity.

"Seeing all these experts agreeing to what you've found is truly special. With the support of this vast body of knowledge, anybody contesting the painting would clearly represent a minority," Six said.

Until now, the existence of the painting had been completely unknown, as there was no previous literary reference to it. This makes the discovery different from other paintings attributed to Rembrandt over the years, as they were already known to exist.

But Six says he knew exactly what he saw when he laid eyes on the painting at Christie's.

"I saw so many details pointing in Rembrandt's direction, that I was totally convinced," he said.

The newly discovered Rembrandt, measuring just under a metre high, is thought to have been painted when the artist was 28. It was almost certainly cut out of a larger painting, experts say, probably also depicting the young man's wife.

Six said he will now try to find a buyer for his discovery, but he did not want to speculate on how much it might be worth.