This is the very renown and legendary super-wide-angle lens made by Carl Zeiss Jena DDR. This lens came in many diiferent versions. There is the older 20 mm Flektogon with a maximum aperture of 1:4 and without multi-coating (MC) and there was also an “electric” version with electronic contacts made for a special Praktica camera.
The lens is known for its excellent optical performance and is still very much sought after on the used market. When stopped down to f/5.6 or beyond, the performance is really outstanding. There is also very little distortion present. If you want to get this lens to use it, make sure you get the version with mulit-coating (MC) and without the electronic contacts.