Clarinet concerto in A major K 622 by W A Mozart
From Wikipedia:
The work was completed a few weeks before the composer’s death. It was to be his last completed work, and has been described as his swan-song. The date of its first performance is not certain, but may have been 16 October 1791 in Prague.
The concerto was written to be played on the basset clarinet, which can play lower notes than an ordinary clarinet, but after the death of Mozart it was published with changes to the solo part to allow performance on conventional instruments. The manuscript score is lost, but from the latter part of the 20th century onwards many performances of the work have been given on basset clarinets in conjectural reconstructions of Mozart’s original.
Enjoy, Jack