Author tracks down 1920s print of painting destroyed during war, and ownership of second work not seen in public since 1948

One day in Arles in August 1888, Van Gogh was planning to paint from life. But the models he had hired failed to show up, and a harsh, hot mistral was blowing, making conditions for painting outdoors unbearable.

So he improvised: he took bunches of Provençal sunflowers, then at their golden-blooming best, and arranged them in locally made, half-glazed earthenware pots. He started work on Monday morning and by Saturday he had made four sunflower pictures.

Two of these are now among the most beloved, celebrated and valuable paintings in the world: they hang in Munich and in the National Gallery, London. Two are lost to public view – one was destroyed in an American bombing raid on Japan during the second world war, the other vanished into private hands after it was exhibited in Ohio in 1948.

Now fresh details have emerged about the lost paintings. The Van Gogh expert Martin Bailey has tracked down a previously unknown 1920s print of Six Sunflowers – the work that was destroyed – so that for the first time since the war it can been seen in its original bright, vibrant colours, and with a hitherto unseen original frame that Van Gogh painted to complement the colours of the subject.

In addition, Bailey has tracked the "missing" painting, charting its progress through private hands after the war to being sold in the 1990s to a "very discreet, private collector" who owns a handful of Van Goghs.

Bailey, who publishes his research this week in a new book, The Sunflowers Are Mine, found the 1920s image of Six Sunflowers in a small museum in Japan, tucked away in a portfolio of Cézanne prints.

One other, much poorer quality, reproduction exists, lacking the detail of the frame. It is also in much less vibrant colours than the print unearthed by Bailey, which matches the description that Van Gogh wrote of the work in a letter to his brother, Theo.

The orange frame – bright where it follows the sky-blue background of the picture and paler where it meets the lilac of the table on which the flowers sit – was "a revolutionary idea in 1888 when the work was painted," said Bailey. "Paintings were normally hung in gilt frames, or for very modern works in white-painted frames … Van Gogh clearly meant it as an integral part of the work."

The painting met its doom on the same day that Hiroshima was destroyed, in a separate bombing attack on Ashiya. Long since removed from Van Gogh's painted frame, which he had had run up by the local carpenter in Arles, it was framed in elaborate gilt and hung above the sofa of a wealthy collector, Koyata Yamamoto. As fire engulfed the house, the large frame made the picture too heavy to rescue.

Bailey has charted the lives of the four sunflower paintings, as well as the three copies Van Gogh made later of two of them, which now hang in museums in Philadelphia, Amsterdam and Tokyo.

They began as pictures the artist made to decorate Paul Gauguin's bedroom, and could not sell, and ended up among the most revered and expensive paintings ever.

Bailey believes that a sunflower painting would, if it reached the open market, make "way beyond £100m". The National Gallery sold 26,000 postcards of its version last year.

The paintings have remarkable histories. During the war Fifteen Sunflowers, the version that hangs in the National Gallery (it actually belongs to the Tate), was sent for safe keeping to Muncaster Castle in Cumbria, high above the old Roman port at Ravenglass. From there it was sent to the Trossachs to receive attention from a German picture conservator who had fled the Nazis.

In his farmhouse north of Glasgow, the conservator replaced a gloomy Rembrandt on one wall with the cheerier sunflowers picture.

Alongside was "Van Gogh's Yellow Chair, a wonderful late Turner and Whistler's Waterloo Bridge, not a bad little collection to brighten up our wartime exile!" he wrote. Wartime conditions meant he lacked proper tools, so when he relined the canvas of Fifteen Sunflowers he used a cheese grater to distribute wax on the back of the work, a domestic iron to spread it and a dentist's burnishing implement to smooth lifting paint on the front.

"They are methods that would horrify today's conservators," said Bailey, "but the painting hasn't needed very much work since."