Mark - I know that Alcatraz falls under that group and three of the Great Lakes Stamp Collection(St. Joesph's North Pier, Spectacle Reef and Thirty Mile Point) all had a letter prefix before their numbers for at least a year before it was decided to remove them. But these were released as limited editions and were not catagorized as open editions by Harbour Lights. In fact all of these releases also had a "400" number assigned to them until Harbour Lights decided to change the number and get rid of the prefix.