It has a back, because the final picture with the A.S.A. film speed indicator on it is the bottom of the back, it slides off like the back the Zeiss Contax that the Nikon SP is a copy of and the Nikon F. It was very common practice with German and other European lenses to mark them in cm in those days, and this made them more like them, because in those days the Japanese were trying to get their foot in the door of western markets by supplying their manufactured goods of high quality at cheaper prices than western manufacturers could to compete with them.