An ink stick for a solid ink feed system of a phase change ink printer includes an ink stick body having an insertion perimeter, and a guide surface with a shaped guide element shaped to interact with an elongate shaped guide rail of the solid ink feed system. The ink stick insertion perimeter is in a plane substantially perpendicular to the insertion direction, and the insertion direction and the feed direction are different. The insertion perimeter has at least one perimeter section forming a nonlinear key element that matches in size and shape a nonlinear key element in the perimeter of the key plate insertion opening. In particular implementations, the insertion perimeter forms a visually recognizable symbol, such as an alphanumeric character. In particular implementations, a plurality of ink sticks form a set in which the visually recognizable symbols differ from one another, and may form a pattern of symbols, such as a sequence of alphanumeric characters.