This is an A12 Film Magazine, the component that allows the Hasselblad 500C to be so incredibly versatile. The "A" stands for "Automatic," meaning it automatically stops at the first frame when you load the film (unlike earlier versions that required looking through a red window on the back). Having multiple backs is a hallmark of professional photography; it allowed a shooter to swap from color to black-and-white film mid-roll simply by sliding in a "dark slide" and switching magazines.