the cassette hereinafter described in detail is used to package bookbinding strips . strips 16 shown in fig1 and fig2 have a base 17 which is a thin narrow elongated strip of thermoplastic material from which project integral studs 18 in the case of male strips , or , in the case of female strips , holes are found at intervals spaced the same distances as the studs 18 . such strips are described in detail in u . s . pat . no . 4 , 369 , 013 or u . s . pat . no . 4 , 674 , 906 . on either side of the cassette are side rails 110 which are of a length to accommodate a plurality of strips 16 . extending the entire length of each side rail 110 are upper 20 and lower 100 longitudinal guides , between which is an inward facing channel 50 having the width of the thickness of base 17 and deep enough to receive the ends of strips 16 . as best shown in fig7 a slot 71 is formed in one end of each side rail 110 and a retainer detent 73 prevents strips 16 from being forced out of the channels 50 . such detent 73 closes off the end of channel 50 . a lever 130 connected to the side rail 110 above the slot 71 may be bent from the solid line position of fig7 to the dot and dash line position , thereby bending the detent 73 out of the way of channel 50 and permitting the strips 16 to be discharged from the channels 50 . it will be understood that when the cassettes are used in automatic machinery , when the cassette is inserted into the machine , an abutment ( not shown ) contacts the lever 130 and bends it down to the dot and dash line position of fig7 so that the strips 16 may be fed out of the cassette for assembly with perforated sheets to be bound . to load the cassette , the detent 73 is moved aside so that the strips 16 may be slid into the opposed channels . the end of each channel 50 opposite detent 73 is blocked by a permanent stop 120 which prevents of strips 16 from falling out of the back end of the cassette . this is illustrated in fig1 . as an option , the solid wall 121 at the end of the channel may be used for that purpose instead of stop 120 . at locations spaced inward but adjacent to the ends of side rails 110 , and integral with side rails 110 , are upward projecting stacking struts 23 and downward projecting stacking struts 25 , shown enlarged in fig1 and 14 . at the respective ends of these upward projecting stacking struts 23 and downward projecting stacking struts 25 are mating stacking means 24 and 26 . in the preferred embodiment , these stacking means 24 and 26 are upward projections 24 and stacking channels 26 , as illustrated most clearly in fig1 , 13 - a and 14 . stacking channels 26 are formed by downward projecting stacking struts 25 comprising of a web 28 and two flanges 29 , such that corresponding projections 24 fit snugly within each stacking channel 26 , as illustrated in fig1 - a . when strips 16 of a male type are to be packaged the portion of the upward projecting stacking struts 23 beneath upward projections 24 provides protruding shoulders 22 upon which web 28 and flanges 29 of downward projecting stacking struts 25 may rest . slanted tops 21 are also located on upward projections 24 to help in guiding downward projecting stacking struts 25 into place . it will be understood that other types or combinations of mating stacking means may be used , so long as the stacking means 24 atop each upward projecting stacking strut 23 mates with the stacking means 26 at the bottom of the downward projecting stacking strut 25 of the cassette directly above it , as illustrated in fig3 and 13 - a . for ease of manufacture and use , it is preferable for all the upward projecting stacking struts 23 have stacking means 24 of one type and all the downward projecting stacking struts 25 to have stacking means 26 of a corresponding mating type , but it will be understood that other combinations may be used as well . in the preferred embodiment , the distance between the top end of an upward projecting stacking strut 23 and the bottom end of the downward projecting stacking strut 25 directly beneath it is only slightly greater than the height of the strips 16 being packaged , from base 17 to top of studs 18 . this allows enough room for studs 18 when strips 16 of a male type are packaged and the cassettes stacked one atop the other , as illustrated in fig1 . the length of the downward projecting stacking struts 25 are designed such that when strips 16 of a female type are packaged , the length of the upward projecting stacking struts 23 needed is a minimum , so that the upward projecting stacking struts 23 consist substantially of just the corresponding stacking means 24 , as illustrated in fig4 . gussets 32 extend from lower guide 100 to downward projecting stacking struts 25 in the cassette for female strips shown in fig1 to prevent channel 50 from closing from shrinkage when cooling after molding . to prevent struts 23 from bending from shrinkage when cooling after molding , gussets 31 are formed on the outer surfaces of struts 23 , tapering upwardly . extending transversely of the cassette adjacent but spaced inward of either end of side rail 110 is a spacer bar 120 . spacer bar 120 has in the preferred embodiment a flat thermoplastic base 155 and , positioned perpendicular to the base 155 and extending longitudinally along the base 155 almost but not quite to the ends of the spacer bar 120 , a flat thermoplastic rib 156 . the rib 156 may be located centrally along the spacer bar base 155 , but it is not necessary that this be so . in the present embodiment , rib 156 is located closer to one side of the base 155 than on the other , as shown in fig1 . rib 156 gives added stability to spacer bar 120 , and in the preferred embodiment serves the added function of supporting base 17 of strip 16 at the end of the cassette opposite that from which strips 16 are fed . in the preferred embodiment , this is accomplished by placing side rail connecting means 19 somewhat above channel 50 and near the end of the cassette opposite that from which strips 16 are fed . this arrangement is illustrated best in fig2 . 9 , 10 and 12 . at the opposite end of the cassette , the end from which strips 16 are fed , side rail connecting means 19 are placed somewhat below channel 50 so that rib 156 does not impede strips 16 as they are fed from the cassette . the spacer bar 120 has at each end connector means 150 for connecting with side rail 110 . in the preferred embodiment this connector means 150 comprises a prong assembly 150 with three prongs , the outer two 151 of which are flexible and are equipped with outward facing tangs 153 at their distal ends . the two outer prongs 151 bend inward as the prong assembly 150 is pushed into a cooperable socket 19 in a side rail 110 , and spring outward once the tangs 153 have passed a shoulder 91 integral with said socket 19 , at which time the tangs 153 engage the socket shoulder 91 and thus secure the spacer bar 120 from pulling out . the center prong 152 is designed to provide rigidity to the prong assembly 150 and to keep the outer prongs 15 1 from breaking or being otherwise damaged by limiting their bending too far inward . shoulders 154 extend on either side of the prong assembly 150 at its proximal end to keep the prong assembly 150 from moving farther into the socket and thereby accurately spacing the opposed rails 119 apart . pads 158 on base 155 of spacer bar 120 help to prevent the weight of strips 17 from causing strut 23 to bend upward . extending transversely of the cassette adjacent but spaced inward of either end of each side rail 110 is the connector means 19 for connecting the side rail 110 with a spacer bar 120 . this connector means 19 is designed to be cooperable with connector means 150 belonging to spacer bar 120 , so that the spacer bar 120 may connect firmly with the side rail 110 . in the preferred embodiment this connector means is a socket 19 that the prong assembly 150 of the spacer bar 120 can be guided into , and also includes a shoulder 91 to catch tangs 153 of the outer prongs 151 and so secure the spacer bar 120 from pulling out . in the preferred embodiment sockets 19 are located at opposite ends of the cassette , either near the top or near the bottom of side rail 110 , as discussed above . as is best illustrated in fig9 - 12 , in the current embodiment of the invention the socket 19 is a hollow rectangle , its walls converging . the prong assembly 150 of the spacer bar 120 is pushed into the inner opening of the socket 19 , its outer prongs 151 being pressed inward by the converging walls as they approach the outer opening . the shoulder 91 in this case is the wall of the outer opening of the socket 19 , so that as the tangs 153 of the outer prongs 15 1 clear the outer opening of the socket 19 they spring outward , thus securing the spacer bar 120 from pulling away from the side rail 110 . it is anticipated that many other cooperable means may be used to secure the spacer bar to the side rail 110 . an advantage of letting the tangs 153 protrude from the outer opening of the socket 19 as in the preferred embodiment is that the tangs 153 may be squeezed together so that the spacer bar 120 may be easily withdrawn from the socket 19 and exchanged with spacer bars 120 of differing lengths , for use with strips 16 of corresponding lengths . in the preferred embodiment of this invention , outward extending ears 15 are located on each of side rails 110 , whereby the cassette may be guided and positioned correctly within a bookbinding machine . a rib 27 is positioned beneath each ear 15 , integral with and perpendicular to the ear 15 and the side rail 110 . rib 27 provides support and stability for the ear 15 . reference will now be made in detail to the preferred embodiments of the invention , examples of which are illustrated in the accompanying drawings . while the invention will be described in conjunction with the preferred embodiments , it will be understood that they are not intended to limit the invention to those embodiments . on the contrary , the invention is intended to cover alternatives , modifications and equivalents , which may be included within the spirit and scope of the invention as defined by the appended claims . the foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description . they are not intended to be exhaustive or to limit the invention to the precise forms disclosed , and obviously many modifications and variations are possible in light of the above teaching . the embodiments were chosen and described in order to best explain the principles of the invention and its practical application , to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated . it is intended that the scope of the invention be defined by the claims appended hereto and their equivalents .