You can see a scan of the prequel at right.
The 800-word story, penned by the author over two sides of an A5 postcard, was a prequel to the bestselling series of books. It was auctioned off nine years ago to raise money for the English PEN, a global literary network that works to "defend and promote freedom of expression, and to remove barriers to literature." As Rowling explained in a tweet, the anonymous owner "supported writers' freedoms" by bidding for the manuscript. The story sold for £25,000 at the auction house Sotheby's, according to the Birmingham Mail.
The thief also stole jewellery from the property, in the British city of Birmingham, which was broken into sometime between April 13 and 24. Officers are asking supporters of Rowling's work to share the appeal in the hope that it "goes far and wide among Harry Potter fans throughout the world," a West Midlands Police statement said.
It's wrong to say that the story was stolen; that wasn't the only copy. Yes, it's the only physical copy, but you can find a transcription on one or more Harry Potter fan wikis.
As you can see from the above link, this is less a story than it is a vignette detailing a brief encounter between Sirius Black and Harry's father, James Potter, and a couple constables. It's signed "From the prequel I am not working on--but that was fun! -JKRowling 2008".
While the loss to fandom was minimal, the collector's loss has to be galling. Let's hope the thieves realize the value of what they stole, otherwise they might think it was only a postcard and throw it away.