Abstract: An improved ink cartridge mounting system for a primary ink jet printer including an ink cartridge mounting element having four cartridge receiving openings, each specially configured to receive an ink cartridge containing ink of a particular color. A unique arrangement of six ribs extends from the outer periphery of each cartridge receiving opening to define a distinctive keying pattern adapted to seat an ink cartridge having a fluted end portion configured to mate with the distinctive keying pattern. The unique keying pattern designated for each different color are entirely complementary to a prior keying pattern employed for the same color in an alternate type of ink jet printer but for the provision of at least one additional rib.