This sounds simply too unbelievable to be true, but it's been verified by Harold Rosson, a cinematographer who worked on the movie. There's something really very remarkable about the jacket worn by Professor Marvel in the film.
When they were designing the professor's costume, the production team decided they wanted something that was clearly once elegant and stylish, but now battered by time and frayed round the edges, much like the professor himself. They found the exact item that they wanted in a thrift store. It was only some time later that someone examined it more closely and found the name of the original owner sewn into the fabric. It was L. Frank Baum. Somehow, they'd walked into a thrift store and accidentally bought a jacket which used to belong to the same man who'd written the story they were making a movie of. What are the chances of that?