 The first, "Clipper Liberty Bell" (N6518C), inaugurated Pan Am's all-tourist class "Rainbow" service between New York and London on May 1, 1952 to complement the all-first "President" Stratocruiser service.From June 1954, DC-6Bs began replacing DC-4s on Pan Am's internal German routes.Pan Am introduced the Douglas DC-7C "Seven Seas" on transatlantic routes in summer 1956.