Inside was an oil painting on wood, only a little larger than a postcard, of a young woman in an elaborate, high-collared dress, its puffed upper arms studded with rubies set in gold. She was wearing a pearl hairnet and a matching necklace; and with her curiously elongated right hand she was supporting a lap dog with bulging black eyes. Her face was expressionless, its only striking feature a distinctly aristocratic nose.
I asked who she was. My benefactor had no idea. All he could tell me was that he got the portrait from a Red Army soldier in exchange for two sacks of potatoes needed to make vodka.
"And don't ask me to keep the picture. I've been through two checkpoints to get to West Berlin and if I'm searched on my way back home ..." Soon afterwards he left.
This was in 1952. I was the Berlin correspondent of the BBC's German language service, working mainly for its nightly programme aimed at listeners in the Soviet occupation zone. My visitor was an elderly farmer from Frankfurt on the Oder, one of several occasional callers at my office in the British sector, who would drop in for a coffee and a chat and contribute to a popular feature we called Briefe Ohne Unterschrift or Letters Without Signature.
West Berlin in those days was being swamped with refugees from communist rule and I had promised my farmer that I would give back his painting if he ever came over to the west. But he never reappeared. The picture travelled with me for the next 54 years, from Berlin to London, to Delhi, back to Berlin, to America, to Brussels and finally, in the 1970s, to our home in England.
Earlier this year, an opportunity came up to show my painting to an expert. Anne Webber, co-chair of the Commission for Looted Art in Europe, has spent the past seven years tracing artworks stolen by the Nazis and persuading those who hold them - museums, auction houses and private collectors - to return them to their true owners, mainly Jewish families. So far, the commission has recovered more than 550 works.
Webber soon came up with a result. Within weeks of sending photographs to three German museums and archives, she had discovered that my mysterious miniature was a 16th-century portrait of Eleonora of Toledo. Eleonora was the Spanish-born consort of the ruler of Tuscany, Grand Duke Cosimo I de' Medici, and mother of his eight sons, two of whom became grand dukes in their turn. The painting - attributed to Alessandro Allori, foster son of the Florentine court painter Agnolo Bronzino - had been acquired by Berlin's Kaiser Friedrich Museum (now the Gemäldegalerie) in 1894. In 1944, it disappeared. However, a black and white photograph, taken in 1939, had survived the war, and was included in the museum's catalogue of more than 400 losses. A page from that catalogue, faxed to Webber by the Gemäldemuseum's curators, was proof enough: there, in the top left-hand corner of the page, was a postage-stamp sized reproduction of "my" Eleonora, together with the museum's inventory number and a brief attribution to Allessandro Allori. It was an exact match. Clearly it was time for a flying visit to Berlin.
How the painting was "lost" is still a mystery. That it was posted missing as early as 1944 suggests it was not among the loot taken by the Red Army's trophy teams and transported to the Soviet Union by the trainload in the summer and autumn of 1945. It may well be that its theft was an inside job - a simple case of a lower-level museum employee pocketing an easily portable object during the bombing of Berlin in 1943, when many paintings and sculptures were taken down for storage in the cellars of the Reichsbank and in Berlin's two multi-storied flak towers. The first, at the Zoo in the British sector of the city, contained the Schliemann collection of Trojan gold, and was cleared by the Russians before the arrival of western troops in July 1945. The most important, oversize paintings were stored in a second bunker, at Friedrichshain in the Soviet sector.
What happened to its contents is a mystery to this day. Although the Russians secured the tower within 48 hours of the end of the fighting, it was looted by civilians looking for food and twice set on fire. The second fire, accompanied by an explosion that demolished a reinforced concrete staircase, has been attributed by a former East German curator of the Gemäldemuseum to an SS "Werewolf" unit carrying out Hitler's so-called "Nero Order" of March 1945, in which nothing but scorched earth was to be left behind. There is no direct evidence of this, however.
What is indisputable is the Gemäldegalerie's loss, at Friedrichshain, of 441 major paintings, among them seven works by Peter Paul Rubens, three Caravaggios and three Van Dycks. What cannot be excluded, at least until Russian curators reveal what works of art they are holding in secret depositories, is the possibility that some of these paintings are still in Moscow and St Petersburg today.
We returned the miniature at a small ceremony in the Gemäldegalerie's library on June 1, a little more than 60 years after it vanished. It was only then that we found out that this is the first time the museum has recovered a work of art since it catalogued its missing paintings in 1995.
The curators hope others may follow. Meanwhile, they expressed sympathy with Webber's suggestion that the return of the picture might act as an example to the many German museums still holding art stolen by Nazis between 1933 and 1945.
For me, this is a happy ending to a brief but intriguing episode in the painting's 400-year life. There is a plan to exhibit Eleonora's portrait, together with other works of Florentine art of the same period, in one of the theme rooms of the magnificently restored Bode Museum, which was destroyed in the war and reopens in October. Eleonora will be where she can be seen, in a far better home than the top of my overcrowded bookshelves in Sussex.
· Charles Wheeler's documentary, It's My Story: Looted Art, is on Radio 4 tomorrow at 8pm