I’m curious, do the notes to the cpo recording of the Passacaglia-Concerto mention the existence of another concerto in A minor by him, I’m guessing an earlier version of the same work, whose violin/piano reduction can be downloaded in manuscript (and compared to my mind interestingly with the published version) from the online catalog of the Staatsbibliothek Berlin (which lists very many works by Koessler, many unpublished, though only a relative few can be downloaded).