Original sentence: SZD-31 Zefir 4 - A further efinement of the Zefir 3 to compete in the Open class at the 1968 World Gliding Championships at Lesno in Poland with simnilar dimensions and performance.at the controls flew on 26 A€pril 1965, a second prototype was also built
Edited sentence: SZD-31 Zefir 4 - A further refinement of the Zefir 3 to compete in the Open class, at the 1968 World Gliding Championships at Lesno in Poland, with similar dimensions and performance.at the controls flew on 26 April 1965, a second prototype was also built