Mary had heard of the Men of Letters, but she thought they were a myth the American Chapters were wiped out right around the time she was born.The British Men of Letters have been watching the Winchesters for years.Lucifer is jumping from vessel to vessel (and possibly running from Crowley).She also doesn't remember Dean's first visit back in time, although her memory was only wiped the second time he returned with Sam. Mary remembers dying, but doesn't remember being a ghost trapped in the house in Lawrence (Season 1, 'Home').This is the first time we've seen what actually happens to an angel who gets blasted away by one of the blood sigils.