In office equipment such as printers and copiers, a single machine "platform" is available as a number of distinct variants (copier, printer, facsimile, MICR printer). Different variants may require different types of marking material cartridges, or other replaceable units. To prevent the installation of an unsuitable cartridge in a machine, each cartridge defines a distinguishing configuration of convex or concave engagement structures, arranged in a circle, but whereby no more than one-half of the circumference of the circle is taken up by the engagement structures. The engagement structures engage with complementary structures inside a suitable machine, and can assist in driving a rotation of the cartridge within the machine.