This is the 'alternate' version of the Kegelstatt, substituting a violin for the original clarinet part. The transcription was provided by the publisher (presumably to increase sales), possibly with Mozart's consent.
The purported story of the duos for violin/viola is that Michael Haydn was assigned the task of composing a set of six such pieces for the Archbishop of Salzburg, but due to illness was only able to complete four in time. As a favor Mozart provided the additional two duos. One detail that could conflict with this anecdote (from pupils of M. Haydn) is that Mozart's two compositions were published in 1782, one year prior to the four by Haydn.
For those who would like to compare the duos from both composers, M. Haydn's are catalogued as MH 335-338 / P120, P127-129