through an opening in the exterior housing of the magnetic tape cartridge for use by an associated tape drive to retrieve the magnetic tape from the magnetic tape cartridge a tape threading arm in the tape drive grasps the leader block to extract the magnetic tape through the opening in the one corner of the magnetic tape cartridge and transport the leader block with its attached magnetic tape along a tape threading path to be inserted into the takeup reel of the tape drive the external dimensions and architecture of the 3480 type magnetic tape cartridge are an industry standard as is the tape threading arm leader block pin that is used to grasp the leader block the leader block illustrated in fig1 5 interfaces to equivalent elements and details of the magnetic tape cartridge and tape threading arm leader block pin are omitted for the sake of brevity as shown in fig1 the leader block 101 in the magnetic tape cartridge 100 is withdrawn into the opening 121 at the corner of the magnetic tape cartridge 100 by the rewind action of the tape drive not shown which rewinds the magnetic tape 108 on the reel 115 contained within the magnetic tape cartridge 100 a stop mechanism not shown is provided in the magnetic tape cartridge 100 in order to prevent the leader block 101 from being fully retracted within the housing 117 therefore in a rewind operation the magnetic tape 108 is wound a sufficient distance to snugly place the leader block 101 up against the leader block stop not shown within the cartridge housing 117.