William S. Parsons was the first Navy officer to be assigned to Los Alamos, General Groves explains (in Now It Can Be Told: The Story of the Manhattan Project), and he appeared at the gate wearing his Navy summer uniform:
His arrival was announced by a frantic guard, who telephoned his sergeant: “Sergeant, we’ve really caught a spy! A guy is down here trying to get in, and his uniform is as phony as a three dollar bill. He’s wearing the eagles of a colonel, and claims that he’s a captain.”