More than 400 years ago, the up and coming Flemish painter Peter Paul Rubens toured the streets of Rome, notebook in hand, sketching images from Renaissance works adorning the city’s churches and palazzos.

Now a rare sheet, thought to be from his Roman sketchbook, has gone on display in his home city of Antwerp, shedding new light on the baroque master.

Unveiled to the public for the first time is a unique double-sided sheet featuring a drawing on one side and a partial draft letter on the reverse. The curator of the Rubenshuis museum, An Van Camp, thinks it probably came from a sketchbook that Rubens used during his time in Rome, where he lived with his brother Philip near the Spanish Steps.

On one side is a quick, spontaneous sketch in brown ink of three men in classical robes, thought to be apostles. At the top of the page are a few thick jagged lines, assumed to be Rubens testing his quill pen. The three figures are not known to appear in any Rubens work and it is not clear whether the artist imagined them or copied something he had seen in Rome.

The reverse is an incomplete draft letter to an Italian painter, Cristoforo Roncalli, who had been commissioned by their mutual patron, Eleonora de’ Medici, the duchess of Mantua, to paint a work for her private chapel. Rubens was employed as a painter to the Mantua court between 1600 and 1608.

In the letter, which is dated September 1607, Rubens, then 30, asks the older artist about the painting’s progress on behalf of his “most serene mistress”. Van Camp imagines it must have been an awkward conversation. She said: “In this draft letter, he is really trying to find the right words, the right tone to be diplomatic, to not offend the Italian painter, but also to make it very clear that he really needs to check the painting for the duchess and that time is running out.”

The handwriting was quite sloppy, she said, with corrections as Rubens searches for words. “The letter is really exciting because it offers a further glimpse into Rubens as a budding diplomat.”

The double-sided sheet is a precious addition to the Rubenshuis, which holds only two drawings and 10 letters by Rubens, all from a later period. The piece can be viewed at the Rubens Experience, an interactive exhibition at the museum, but is destined for displayed in his Antwerp home when it reopens after a extensive renovation in 2030 at the earliest.

Very few of the old master’s works remain in Belgium. Many of his paintings went to Madrid and Vienna, taken by the Spanish and Austrian imperial rulers who successively controlled the territory of present-day Belgium in the early modern period. Rubens’ drawings were highly sought by English and French collectors in the 18th and 19th centuries, further dispersing his work.

The notebook sheet was acquired for Belgium by the King Baudouin Foundation (KBF) for €110,000 (£86,000), a reduction on the asking price of €145,000 agreed by the private US owner because it was destined for public display. “This [purchase] request ticked all the boxes,” said Melanie Coisne, the head of KBF’s heritage and culture programme. Not only was it a rare piece by a renowned Antwerp artist but it offered “a real insight into who Rubens actually was”, she said.

Rubens left Italy in 1608 on learning his mother was gravely ill. He arrived in Antwerp too late to see her before she died, but settled there as big commissions began to trickle in. But he never forgot his Italian years, signing as Pietro Paolo Rubens for the rest of his life.

Yearning for Italy, he turned his Antwerp town house into a palazzo inspired by ancient Rome and the Renaissance. “His heart was still in Italy, but he never returned,” Van Camp said.