Manuscript Am.B. 340 was presumably written by Wilhelm Karges. As was usual in those days composers were indicated with their initials rather than there full names. Or were not indicated at all. If Karges indicates no composer then we can only assume the piece was composed by Karges himself or by an anonymus master.
As composer for the Capricio, notated on page 44 and 45 of the manuscript, Karges indicates "J.P" as the composer. The most logical assumption would be that these initials stand for Jan Pieterszoon Sweelinck. The music is build on ascending and descending chromatic lines. It is, however, not an adaptation of Sweelinck's famous Fantasia Cromatica. Or if it is, Karges only used elements and creates his own composition with these elements. It seems more likely he used another composition by Sweelinck to adapt to this Capricio. Again, if someone knows on which original Karges based this music, I'd very much like to hear it.
The music is very well written. The chromatic lines create a lot of tension that in every case resolve beautifully. The bass part feauture some very long chromatic lines, in one case even spanning a decime. Karges created a beautiful composition, about half as long as Sweelinck's Fantasia Cromatica. And that's probably why he did it: in his practical duties as church organist, he needed music of about 3 to 4 minutes in length. And not 8.
Score available here: http://partitura.org/index.php/wilhelm-karges-j-p-capricio-in-a