However, since several notes throughout the piece go beyond the conventional range of the A clarinet, we can presume it was intended to be played on the basset clarinet, a special clarinet championed by Stadler that had a range down to low (written) C, instead of stopping at (written) E as standard clarinets do.
Even in Mozart's day, the basset clarinet was a rare, custom made instrument, so when the piece was published posthumously, a new version was arranged with the low notes transposed to regular range. This has proven a problematic decision, as the autograph no longer exists, having been pawned by Stadler, and until the mid 20th century musicologists did not know that the only version of the concerto written by Mozart's hand had not been heard since Stadler's lifetime.