Wasn't there a "mechanical" difference in the instruments of today and those used by the orchestra in Mozart's time? Particularly with the French Horn and back in Mozart's day the Horn player had to swap bits of the horn. There was a particular virtuoso horn player who Mozart used to try and catch out, this sort of duel was known to the audience who were known to burst into applause when the horn player managed to "pull off" the changes required to play the piece of music.