If you think that your rangefinder lenses are small, you ain't seen nothing yet! In Japan, a dedicated lens maker Mr. Miyazaki set-up his own company called MS-Optical to create astoundingly small lenses for rangefinders. He gained fame a few years back with his first product - the Perar 35mm lens, which really looks more like a body mount cap than a lens!
Now he has pushed the boundaries further with a new lens - the Perar 28mm f/4 lens, which is even smaller than his debut product! What's really amazing is that all his lenses are handmade by him in limited numbers, which explains their rarity and scarcity. They are popular for delivering good performance in an ultra-compact package.
You can order the lens from here.