That postcard ended up selling for £25,000 ($32,152) at a charity auction in 2008, complete with a "From the prequel I am not working on - but that was fun!" note at the end.
The story is said to feature a young James Potter, Harry's father, and his best friend Sirius Black. In the story, the two wizards are caught by two Muggle policemen after a high-speed motorbike chase - but in true form, the boys manage to fly off on their broomsticks.
West Midlands Police issued an alert and call for information in the hope that "the appeal goes far and wide among Harry Potter fans throughout the world".
The owner of the prequel, who does not wish to be identified, thanked JK Rowling and the police for raising awareness about the theft. Keep an eye out for the stolen manuscript and report it to police immediately if you find it - no matter how willing you are to empty your Gringotts vault to buy it.
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Rowling penned the brief prequel to her wildly popular Harry Potter series in 2008 for an auction held by English PEN - a foundation that promotes the freedom to write and read. "We are appealing to anyone who sees, or is offered this item for sale, to contact police", investigating officer PC Paul Jauncey said in the statement.
The handwritten postcard by @jk_rowling was purchased to raise money for charity. Rowling sent out a tweet begging people not to buy the Harry Potter prequel if they are presented with the opportunity.
But I took good care of it. With no leads, the West Midlands Police have turned to the public to hopefully track down the thief and recover the stolen property.
The second magical tale from J.K. Rowling's will be brought to life at 3Arena Dublin on December 6.
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