The original source of this piece in manuscript Am B 340 is a manualiter Toccata in D Dorian by Johann Pieterszoon Sweelinck. This Toccata is actually the source for two different pieces. The first piece that is based on this Toccata uses the first 14 bars of this Toccata as a starting point. After those 14 bars the music in this piece differs from the music of Sweelinck. Perhaps this part of the piece (almost 50 bars) was composed anew, or perhaps it was based on yet another model. If someone knows which other piece these 50 bars could be based on I would very much like to hear it. As it is, the piece starts with a slow pace and works gradually to faster passagework. As said already in other posts, Manuscript Am.B. 340, as a source of works by Scheidemann or Sweelick or others, has been called "utterly unreliable". Nevertheless, I very much like these "utterly unreliable" pieces.
Score available here: http://partitura.org/index.php/wilhelm-karges-jan-pieterszoon-sweelinck-fantasia