Abstract: In a general aspect, a system for selectively positioning a work piece includes multiple bodies. A first body includes a first exterior surface and a boss extending from the first exterior surface along a boss axis. The boss includes a boss end surface that terminates the boss. Boss perimeter surfaces define a perimeter of the boss and extend between the first exterior surface and the boss end surface. A second body includes a second exterior surface and a pocket extending from the second exterior surface into the second body along a pocket axis. The pocket is shaped to receive the boss in a discrete number of positions when the boss axis and the pocket axis are aligned. Pocket perimeter surfaces define a perimeter of the pocket and extend into the second body from the second exterior surface.