“Mr. Mozart, Kapellmeister, wishes to announce to the most worthy public the appearance in print of three of his recently contrived piano concertos. These three concertos may be performed not only with a large orchestra including wind instruments, but also a quattro, that is, with two violins, one viola and violoncello. They will be issued early in April of this year...”
As this advertisement shows, it was Mozart’s intention to keep the accompaniment part flexible in his piano concertos K.413-5 and, as we know from a letter Mozart wrote to his father, in K.449 as well.