Ditmas Avenue was once Avenue E; it shares its name with Ditmas Park.
Late 19th- and early 20th-century maps show an Avenue E, later renamed Ditmas Avenue between Dahill Road and Coney Island Avenue. The neighbourhood of Ditmas Park was named after the Ditmas family, who owned the rural land on which it was built. Wikipedia also notes some evidence that today’s Avenue D was originally named Ditmas Avenue.