The Adventures of Jimmie Dale — Frank L. Packard

Jimmie Dale is a wealthy playboy by day, with a Harvard education and membership to New York City's ultra-exclusive private club St. James. But at night he puts on a costume and becomes The Grey Seal, who enters businesses or homes and cracks safes, always leaving a diamond shaped, grey paper “seal” behind to mark his conquest, but never taking anything. He was just doing it for “the sheer deviltry of it” at first, but when a woman catches him, she blackmails him to war on certain crime organizations.