Is it a bald elderly man with a big bushy beard and a wine-addled stare? Or a friendly young woman with flowing locks and a crown of braids?

To Belgian art dealer Klaas Muller, an answer to that question mattered less than the fact that this particular take on the duck-rabbit optical illusion was painted by one Peter Paul Rubens.

Three years ago, the Brussels-based former gallerist managed to effectively acquire not just one but two studies of heads by the Flemish baroque master in one painting, paying the “reasonable price” of less than €100,000 at an online auction.

The auctioneer, which Muller would identify only as a “lesser-known auction house in northern Europe” for fear of encouraging too much future competition, had advertised the work as an undated study on paper by an anonymous master of the “Flemish school”.

In the recent past, studies by Rubens have sold for between £500,000 and £1m or more.

“I wasn’t sure it was a Rubens, I just knew it was very Rubens-esque, so it was still a gamble,” said Muller, who describes himself as a passionate admirer of the 1577-born artist and diplomat. “I have a library of books about him at home and look at them most evenings,” he told the Guardian. “It’s a bit of an addiction.”

He felt more confident when the painting was delivered to his home. “It was very dirty, but the varnish had protected the painting very well and I could see that it was of extremely high quality.”

But it was not until after the picture was studied for several months last year by the art historian Ben van Beneden, a former director of the Rubens House, that Muller started to feel confident he had acquired a genuine old master.

“I think it’s very likely,” said van Beneden. “You have to be cautious because you are dealing with a painting that wasn’t made for the market but as a working material. But the craftsmanship is outstanding – it has a very lifelike quality.”

The old man figure in the study appears in several of Rubens’ better-known paintings. “He is omnipresent and versatile,” wrote the Belgian newspaper De Standaard, which broke the news of the painting’s discovery this week.

In The Raising of the Cross, a raised altarpiece in Antwerp Cathedral, the old man is depicted as Saint Amandus. In The Adoration of the Magi, which hangs in the Prado museum in Madrid, he is a red-cloaked King Melchior. In The Tribute Money, at the Legion of Honour museum in San Francisco, he is the Pharisee peeking out from behind Jesus.

Inspired by Italian painters, Rubens collated a series of different physiognomies he could use for larger paintings.He is known to have created a prototype study of the old man’s head that has since been lost. “Muller may indeed have found the prototype,” van Beneden said.

The woman inside the old man’s beard, however, was painted first. Rather than trying to create an optical illusion, Rubens probably reused the paper of an earlier painting and traced over it, Muller said.

The painting hangs in Muller’s home and will go on display at the Brafa art fair in Brussels on 25 January. Even as a study, the art dealer said, it deserved as wide an audience as possible. He hoped that a museum would agree to take the painting on a long-term loan.