Three figures in classical mantles, probably apostles

From late 1605 until late 1608 Rubens was in Rome as an agent of Vincenzo Gonzaga, Duke of Mantua, for whom he sourced art to add to the ducal collections, including a painting by Cristoforo Roncalli, il Pomarancio (circa 1553-1626) and, more famously, Caravaggio's monumental Death of the Virgin, which had been rejected by its original patron Lorenzo Cherubini. It is not clear to whom the letter on the verso of the present study is addressed: some scholars have suggested that it may be a letter to Pomarancio, although the courtly formality of the language may point to the recipient's being someone in the entourage of the Duke of Mantua. The dating of the letter to September 1607 (not November, as previous transcriptions have recorded) places this and the sketch precisely at the time that Rubens was working most closely with the Gonzaga.

The free and confident figure study on the recto of this sheet cannot be linked to any known composition by Rubens, although the classical mantles of the figures suggest they are apostles. Considering Rubens's interest in Caravaggio at this period, there is an intriguing resemblance in reverse to the composition of Caravaggio's Calling of Saints Peter and Andrew in the Royal Collection, London.

The letter has been transcribed by Herwarth Röttgen as follows:
'... tra l'altre per la memoria... ... [mia?] Sereniss.ma Padrona del suo quadro... [è] restata informatiss.ma di quanto si deve proprio [al] sommo valore di VS lo sentirei grand.mo gusto [veder] nascere colla solita felicità fra le mani sue una cosi [bella?] opera, del resto non sarei qual VS. dice ma disutile.... eccetto in ammirarla. In questa consolacione chio [sono] certo di riceverne mi sarà gran stimulo à venerlo [esami] nare quanto prima. Fra tanto mi conservi nell[a] bona gracia et in quella d' il Sigr Abbate Crescenzi... col mezzo di VS. potrò meritarla. E per fine le ba[cio le mani]...'