City crime novel Frankenberg (Eder)

By a stroke of luck, police officer Stefan discovers the counterfeiting workshop of the Luceat duo. The officer was actually on the trail of another crime when coincidence led him to a certain object. For years, the criminal duo had professionally copied works of art by renowned artists in a dilapidated half-timbered house on Dellbrücke Street and sold them worldwide. It must have been lucrative, because a very considerable sum of cash and numerous
Gold bars found in the house.

During the raid, the police were able to arrest one of the two forgers on the spot. The other is on the run! The apprehended forger, however, refuses to reveal anything, except that among the artworks found is an original. He refuses to reveal which one it is. Even when the police officer questioned him forcefully, the forger didn't respond—he couldn't believe it: Instead, he stuck out his tongue. What a rogue!

Police officer Stefan isn't an art expert, but he recognizes the works. They're all by Philipp Soldan. But which artwork is the original?