Three figures in classical mantles, probably apostles

Sir Peter Paul Rubens (Siegen 1577-1640 Antwerp) Three figures in classical mantles, probably apostles pen and brown ink, lower corners made up 4¾ x 8 in. (12 x 20 cm.) drawn on an extensive fragment of a letter written by Rubens in Italian, dated 'Settembre 1607' (verso)