This may seem a bit like the old joke, “who’s buried in Grant’s tomb?” but an article in the 2004 issue of the Historic Brass Society Journal casts doubt on this being a work actually by Mozart. Cutting to the chase, in his article “The Doubtful Authenticity of Mozart’s Horn Concerto K 412” Benjamin Perl concludes,
…the autograph of the first movement and the Rondo fragment known as the “Horn Concerto K 412+514” are probably not original compositions by Mozart, but a result of brushing up an earlier work by another composer, who may well have been the horn player Joseph Leutgeb. The blatant stylistic divergences from Mozart’s practice, the anomalous two-movement structure, and the curious layout of the autograph permit such an interpretation.
The many dissimilarities … suggest that the second was not an elaboration of the first, but that both were based on an earlier version, which, again, may have been Leutgeb’s, his involvement having left footprints in both manuscripts.
The evidence presented by Perl is compelling. The article itself is based on a paper presented at the meeting of the Mozart Society of America in Columbus, Ohio in 2002, and is certainly must reading for anyone seriously interested in the history of the Mozart horn concertos. He notes at the beginning of the article that he
…approached this research with awe and even some hesitation: the autographs of the Horn Concerto K 412 … exist … and there is no reason to doubt the authenticity of the handwriting.… The very first time I listened to it, however, I had serious doubts that Mozart was its author…. These doubts were reinforced by Alan Tyson’s discovery that the concerto was written late in Mozart’s career, most probably in the last year of his life. Its curious style might be acceptable to a certain degree in an “early” work, but it seemed altogether incompatible with Mozart’s late works.
This article is very worth tracking down in full, he analyzes the topic in depth, including an analysis of both versions of the Rondo movement. That these movements are all reworking a composition by another composer such as Leutgeb is presented convincingly and adds greatly to our knowledge of Mozart and his works for horn.