BWV 639 is one of J.S. Bach's most famous organ works. Lesser known is an extended version of this chorale prelude BWV Anh. 73. It was probably written by Bach's son Carl Philipp Emanuel. He mainly used the original composition and added some elements to it. But maybe it is an original J.S. Bach composition, since Bach often reworked his compositions... Although that's not obvious since the added elements are more written in the style galante, characteristic for C.P.E. Bach's compositions.
For BWV 639 I recommend using headphones, since I only use a 16' in the pedals (there isn't a pedal coupler on this organ and an added 8' in the pedals is too loud).