The diligent Daniel Grant has a valuable backgrounder on photographers edition practices on Huffington Post:
Photographers did not begin limiting editions of their images until the late 1970s, when prices and the market for photography began to increase significantly, as prospective buyers wanted an assurance that there would not be a flood of additional prints that would reduce the value of the works they had purchased. Fourteen states across the country — Arkansas, California, Georgia, Hawaii, Illinois, Iowa, Maryland, Michigan, Minnesota, North Carolina, New York, Oregon, South Carolina and Wisconsin — all enacted print disclosure laws to protect consumers of limited edition graphic and photographic prints (the New York State law also covers sculpture editions). In these laws, the sellers of prints are required to provide buyers with documentation that the artworks they had purchased are part of a limited edition, the number of copies in the edition and that no other editions of the same images exist. Important exceptions, however, are made for earlier limited editions that are of different sizes (a 10″ x 8″ photograph produced in a 5″ x 4″ format), different production techniques (a gelatin silver print produced as a platinum print or a photogravure) or different numbering (Arabic numbers on one edition, Roman numerals on another).