Case of the Cintimani Stone: A Hammer Mystery

Harry Potter-Mason wants nothing more than to be a detective in the Met like his adoptive father. He's immersed himself in Film Noir and the hard-boiled greats, taken to being called "The Hammer" as he tries to find out more about how his parents died. One day a letter arrives telling him that he's a wizard, giving him one answer while asking more questions than the Hammer can count.
Harry finds himself welcomed back to the wizarding world, not simply as one of their own, but as a celebrity. He finds himself 'the boy who lived': the only one to ever survive the killing curse from a villain stylizing himself The Dark Lord Voldemort, who had begun a brutal wizarding war, only to vanish after failing to kill Harry.
In a post-war world, not everything is daisies at the Hogwarts School of Witchcraft and Wizardry either. The Hammer and his newfound friends chase after an item brought in with its own mysteries - a perilous, foolhardy investigation that sets them at odds with their peers and the establishment on their quest for the truth, not only to prevent the item from falling into the hands of evil, but also the secret fate of the man who murdered Harry's parents.

AU Notes: Petunia Evans married a detective from Scotland Yard whilst Harry grew up obsessed with hard boiled film noir detective movies and Humphrey Bogart.