Klein spent his final winter, after he turned 25 on 6 December, in the freezing cold of Poland. As icy winds blew in from the north, he found an opportunity, remarkably, to smuggle out a letter to Prague. The details of exactly how that was achieved are still a mystery, but he was assisted by a prisoner in nearby Lager Waldek, Simon Pastre, whom he may have encountered on work duty. Any other contact between prisoners of the separate camps was impossible, and the prisoners sent from Auschwitz were deliberately and specifically isolated. Some prisoners, though not the Jewish ones in Larger Süd, were responsible for the mail service to and from to the camp, and it is possible that Pastre was involved in this activity in some way. Apart from the Soviet and Jewish prisoners, the ones from other nationalities were permitted to correspond with family, subject to censorship. There was always a risk of being caught for smuggling letters, the punishment being fifteen lashes in public in order to intimidate the other prisoners. But both Klein and Pastre must have felt it was worth the risk, so that the letter could somehow circumvent censorship; and it was, because it contains first-hand information about the dire circumstances of the prisoners which would have normally been expurgated.

The double-sided letter in Klein's hand is largely legible, and signed by him, though the name and address of the sender is that of Pastre, written in the latter's somewhat calligraphic handwriting. It would have been impossible for Klein to receive a reply, but more of a chance for Pastre to do so. Certainly, as the name suggests, and the fact that he had written the French word for address, ‘adresse’, Pastre seems to have been French, but beyond that, his identity remains an enigma. An inventory of the number of prisoners for that winter indicates that Waldek had among its prisoners 91 French, though no Belgians. One wonders whether Simon Pastre might have been connected with the non-Jewish Comtesse Lily Pastre, who helped shelter Jewish musicians in occupied France, as well as helping to preserve the music of Jewish composers by organising clandestine concerts. Conversely, this generous individual might have been using a pseudonym. Knowing that he no longer had family back in Prague, Klein addresses his letter to his sister Edith's mother-in-law, Marie Doláková.