The address “1209 North Orange Street” in Wilmington, Del., has become known in recent years as the epicenter of U.S. corporate secrecy. The squat, split-level building is the official address of over 285,000 companies, many of which are looking to take advantage of Delaware’s Panama-like secrecy rules, tax incentives, and business-friendly case law.In the wake of the recent “Panama Papers” scandal, this unassuming brick office has received renewed scrutiny from the Washington Post, the New York Times, the Telegraph, and advocates for corporate tax reform.But one of its tenants may come as a surprise—a company owned by Democratic presidential frontrunner...