This early Apple mouse prototype was built to test the viability of the optical-mechanical design Hovey-Kelley engineers produced. From an engineering point of view, this was one of most technically challenging parts of the project: the number of mouse buttons, and the ergonomics of the design, were debated more intensively, but neither presented the kinds of mechanical challenges involved in the work represented here.
An early prototype with a simple cover. Note that it has no buttons.