for convenient packaging of plastic bookbinding strips , cassettes which may be securely stacked one upon the other are provided . the end edges of each strip are received in channels formed in opposed side rails . the rails are attached to transverse spacer bars which are preferably detachably connected to the side rails and are interchangeable to accommodate strips of different lengths - e . g ., 81 / 2 inches , 11 inches , etc . cassettes may be securely stacked one upon the other by means of mating stacking struts extending upward and downward from the side rails . very short stacking struts may be used when packaging flat female binding strips in the cassette . a detent is provided in the ends of the rails preventing removal of strips until the detent is bent out of position . the cassette is especially useful in equipment which mechanically assembles punched sheets and strips preparatory to binding a book in a binding machine . projections from the side rails may be used by such equipment to guide and center the cassettes within the machine .