The Plank Wing is a unique design of the mid-1980s by Christof Kratzner of the Germany. It predates gliders such as the Extaccy and Pegasus. It featured composite construction, no leading edge sweep, and a fixed vertical tail to control yaw and slip.
If you have further information, please send it along.
This info (thanks to Hendrik Maroske) is from the 1993 edition of "Wings of Europe" which today is called "WIDOLA"-World Internet Dictionary of Light Aircraft: