Complex sentence: The "world's most famous stamp"--the unique 1856 British Guiana 1c magenta--is cut into an octagon shape and consequently has been referred to as being cut to shape, although technically that term is incorrect as the stamp was originally rectangular in shape.
Split into simpler sentences: The "world's most famous stamp"--the unique 1856 British Guiana 1c magenta--is cut into an octagonal shape. Consequently, it has been referred to as being cut to shape, although technically that term is incorrect as the stamp was originally rectangular in shape.