This is a remix of https://www.thingiverse.com/thing:53451, although the remix search box seems not to be able to find it.

It's a reduction gear based on a planetary gear arrangement that can get fairly high ratios in a fairly compact space, inspired by an enigmatic throwaway comment on the Wikipedia page about planetary gears:

> During World War II, a special variation of epicyclic gearing was developed for portable radar gear, where a very high reduction ratio in a small package was needed. This had two outer ring gears, each half the thickness of the other gears. One of these two ring gears was held fixed and had one tooth fewer than did the other. Therefore, several turns of the "sun" gear made the "planet" gears complete a single revolution, which in turn made the rotating ring gear rotate by a single tooth like a Cycloidal drive.[citation needed]

I couldn't find a citation for it, so I knocked one up in openscad.