It appears that the notation on the reverse side refers to the relic as a piece of clothing belonging to Blessed Pius X. He was beatified on June 3, 1951 (and canonized on May 29th, 1954). The notation was signed on February 17, 1953 by Maria Pia Sartor.
The Controversy Surrounding the Election of Pope Pius X
Pope Pius X was elected but really Cardinal Rampolla received the most votes. He didn’t end up being Pope, because the Archbishop of Krakow used the veto vote of Austria’s Emperor. That was the last time a secular power intervened in a conclave. The year after his election, Pius X abolished that rule under the threat of excommunication. From then on, only cardinals could vote. Papal Artifacts refers you to the story connected to the acquisition of these artifacts, which are some of the rarest items ever to have become part of the Collection. The items are featured under this link.
Pius X was obviously interested in the spiritual well-being of Catholics. Known as the Eucharistic Pope, he not only encouraged daily reception of the Eucharist, but also sought to allow children to receive it at the age of reason (seven years of age) rather than the customary twelve to fourteen years. In 1914, he also revised the missal. In total, his initiatives were so far-reaching he was hailed as a pioneer.
A deeply conservative man who was so transparently good and humble, he was highly regarded for his holiness during his own lifetime. Many miracles were credited to him even while still alive. He died at the beginning of World War I and was devastated at its outbreak. Rather than bless the Austrian troops who sought his blessing, he simply stated, I will bless peace. He was seventy-nine years of age and was buried initially in the Vatican grottoes. In the 1950’s his body was transferred, along with Blessed Innocent XI’s and (eventually) Saint John XXIII’s to three altars in St. Peter’s Basilica where their relics are venerated in glass sarcophagi.