Roger Lebow:

>There is a fragment of a concerto by WAM, from latter Salzburg years, I
>seem to recall.  But it's really only fragmentary, not enough for someone
>to complete.
>
>Mozart also wrote a substantial portion of an Andantino, which HAS been
>completed by several editors over the years.  I believe this is from
>about 1780.  It's a lovely little piece (I've performed it), and the
>cello writing is fairly idiomatic.

An interesting book I recently read -- 'Las Aventuras de un Violonchelo,' by
Carlos Prieto, the dean of Mexican 'cellists -- has this to say on the
subject:

    King Frederick William II of Prussia...  an amateur cellist
    of some distinction, is forever in our debt for having
    commissioned many works for violoncello from the era's main
    composers.  Among them are three of the few works that Mozart
    composed for the instrument: his last three string quartets,
    dedicated of course to the king, in which the cello plays
    an especially prominent part.

    The famous Sinfonia Concertante for violin, viola and orchestra
    was initially conceived as a work which would also have a
    'cello soloist, as a few drafts demonstrate, but Mozart soon
    decided to eliminate it.

    Around 1782, he began an Andantino in B flat major for cello
    and piano, which he abandoned on reaching 33 measures.* It
    is also known that Mozart composed a concerto for violoncello
    and orchestra, K 206a. The manuscript was lost and nothing
    is known about its possible performances during the composer's
    life.+

My translation of an excerpt from page 272.  Prieto refers to: 'The Mozart
Compendium,' ed by HC Robbins Landon, Schirmers Books, New York, 1990: pp
343* and 355+

Bert Bailey