A height-adjustment mechanism may include an integral one-piece leverage body; an integral one-piece sleeve; and a locking member. In an embodiment, the integral one-piece leverage body has a handle, a pair of pivot pins projecting from opposed sides, a tongue projecting rearwardly, and a resilient biasing member projecting forwardly. These parts may be made of low cost materials suitable for integrally forming their features in an injection-moulding operation. Various features built in to these parts may provide a user with a sense of quality.