Single reel tape cartridge with leader block door seal

A magnetic tape cartridge comprising:

• a casing having a generally four-sided, top and bottom, box-like structure with one corner angled at approximately 45.degree. relative to its adjacent sides to form a shorter fifth side;
• a single reel hub mounted for rotation within said casing;
• a length of magnetic tape wound on said hub;
• a generally rectangular shaped leader block having the free end of the magnetic tape connected to a first side surface adjacent to a trailing surface of said leader block, and adapted at an opposite second side surface to the first side surface at a leading surface of said leader block for connection to a device that is adapted for unwinding said magnetic tape from said hub by pulling said leader block;
• and
• a leader block receiving well formed at an angle of approximately 45.degree. relative to adjacent sides at said fifth side of said casing and including a window opening through which said magnetic tape passes as said leader block is moved away from the casing and said receiving well, said receiving well including bottom and side surfaces placed at 90.degree. to each other defining said window opening and further including opposed surfaces forming a part of the adjacent sides of said casing at said fifth side to which said bottom and side surfaces are attached for engaging the leading and trailing surfaces of said leader block in a clamping relationship such that said first side surface and said trailing surface of said leader block cover said window opening to substantially seal said magnetic tape from external contamination.