A first unit comprises a projection having a first planar portion and a second planar portion, which intersect at an angle. The projection is an extension of the first unit, has a linear axis, and is perpendicular to a base end of the first unit. A second unit comprises a mounting face and a receptacle. The receptacle has a shape for receiving and holding the projection. The base end is aligned with the mounting face by inserting the projection into the receptacle. The receptacle engages the first planar portion and the second planar portion, thereby preventing rotation of the projection, the first unit and the second unit about the linear axis.