This is the lens screwed onto a home-made lens board so that my students could use it for experimenting with paper negatives in the back of a 5x4 camera. In my ignorance, I had assumed that the lens was from an old projector or magic lantern, given that it had a fixed aperture, no shutter mechanism and rack-and-pinion focusing. The only thing that made me question such an assumption was that it had a lens hood (removed in the above photo) – not something you would expect on a lens where the light comes out of the lens.
Needless to say, the lens came apart pretty easily. A lens like this is a classic example of form and function co-existing in complete harmony. The function of each part is evident from its appearance. Four screws hold the pinion against the rack while two more keep the rack in place. Other than that, individual parts go together by screwing one into another.