In 2018, Laura Young, the owner of vintage good shop in Austin, Texas, spotted a carved marble bust priced at $34.99 in a local Goodwill. At 52 pounds, it was heavy, and it looked old. An expert examination revealed that it was, in fact, ancient. It dated back to the 1st century CE—and had once belonged to the collection of King Ludwig I of Bavaria. Four years later, the ancient Roman bust, which may depict Roman commander Drusus Germanicus, will be returned to Germany’s Bavarian Administration of State-Owned Palaces, Gardens and Lakes. In a signal of the institution’s gratitude, it...