Markull, Friedrich Wilhelm : Ballade, Kl, op. 66
| Friedrich Wilhelm Markull an Louis Spohr in Kassel | Danzig, Mitte? September? 1856 | 1856091544 |
| Friedrich Wilhelm Markull an Louis Spohr in Kassel | Berlin, Freitag 27. November 1856 | 1856112744 |
| Friedrich Wilhelm Markull an Louis Spohr in Kassel | Berlin, Sonntag 10. Mai 1857 | 1857051044 |