Dutch art dealer Jan Six says he knew right away that a painting catalogued as by an unknown artist, in the circle of Rembrandt van Rijn, was the work of the master himself.

“I saw so many details pointing in Rembrandt’s direction, that I was totally convinced,” he said about previewing the portrait of a man at a Christie's sale.

A particular fashion of 1633 shown in the sitter's collar, painted in Rembrandt's signature style, was one clue. "So at that time Rembrandt just came from Leiden and he didn't have a circle, so I thought what they've written down doesn't make sense and the picture looks too good," noted Six.

With an unnamed investor, Six puchased “Portrait of a Young Gentleman”, painted around 1634, for 137,000 pounds ($185,000) at the London auction in 2016.

Months of analysis and expert opinion was sought before the work was officially assigned to the Dutch master. It is the first Rembrandt to be rediscovered since 1974, and it was completely unknown before now. The portrait will go on view at the Hermitage in Amsterdam for one month.

Six, age 39, grew up with a Rembrandt portrait of his ancestor, also named Jan Six, hung in the family home. He said: "I am completely obsessed with Rembrandt. And to have the ability in your lifetime to find one out of nowhere, it's a dream."

The painting will be for sale, but Six has not disclosed how much he thinks his find is worth.