Four voice fughetta on the chorale, with ad lib pedal added. From the Neumeister Chorale Collection.

The Schmieder II 1990 catalog update says "BWV 1096 Christe, du bist Tag und Licht, oder: Wir danken dir, Herr Jesu Christ (S. 36 f.--Anhang III Johann Pachelbel)."

I have examined a 29-measure fragment by Pachelbel which does seem to be nearly exact except for accidentals, but that doesn't explain the extra 27 measures.