Kevin Sutton wrote:

>Indeed.  Mozart's works were never assigned opus numbers by publishers.

Some of them were.  His first sonatas - for keyboard and violin - were
published in 1764-1766 as (not surprisingly) op.I (K.6,7), op.II (K.8,9),
op.III (K.10-15), and op.IV (K.26-31).  Many years later, two sets of
violin sonatas (six sonatas each) written in 1778--1781 were again
published as op.I and op.II.  The famous six string quartets dedicated
to Haydn were published as op.X, for instance.

>In fact, if memory serves, he didn't see a great deal of his music
>published in his lifetime.

Most likely no more and no less than many of his contemporaries writing in
the same genres.  What got published then was mostly chamber and solo music
- original or arrangements - because that sold easily to amateurs.  Operas
and symphonies were usually distributed as handwritten copies:  the demand
wasn't large enough to warrant publication.  Church music was too tied to
local customs and available forces that it wasn't often published, either.
Mozart certainly cared about having his works published and derived a
significant part of his income from the publications.

-Margaret