This LEITZ SUPER-ANGULON 21mm f/4 is the smallest and lightest 21mm lens ever sold by Leica. It's the size of a golf ball, but because it's solid brass, weighs as much as a larger lens (8.8 oz. or 251g).
This tiny lens is unique, and my favorite, because in addition to its high optical performance, it's the only Leica 21mm lens that takes standard 39mm filters and front caps. Therefore you need not trouble yourself carrying a second set of 48mm, 55mm or 60mm filters and caps just for the 21mm lens, as newer 21mm lenses expect.
This SUPER-ANGULON 21mm f/4 is also the first 21mm lens ever sold by Leica. It actually was made by Schneider, and resold by Leica.
The LEICA 21mm f/4 is an exquisite jewel of a tiny lens, with a magnificent 9-bladed diaphragm and complex 9-element design.
Its performance is excellent. Unlike newer lenses, it has no visible distortion, and it is sharp edge-to-edge. Corner falloff of illumination is as to be expected from a symmetrical ultrawide lens, which means that it's great to be used for its artistic effect and saves you from extra edge-burning. Falloff looks great; it's a smooth fall-away from center which gives punch, like this: