These thirteen sūtras are to be found in most Kangyurs of the Tshalpa tradition grouped at the end of the final, “miscellaneous sūtra” volume of the Perfection of Wisdom division (despite being unrelated to the prajñāpāramitā literature). In other Kangyurs, they are placed in the General Sūtra section—at the end in the Narthang and Lhasa Kangyurs, and in the penultimate volume in most Kangyurs of the Thempangma tradition.
They are called “newly translated” (gsar du ’gyur ba) in the catalogue of the Degé Kangyur, and the same epithet was used by Butön in the 14th century, as well as by Terdak Lingpa and the Lithang Kangyur editors in the 17th century. The Lhasa Kangyur catalogue also calls them “the group of thirteen sūtras” (mdo tshan bcu sum po).
They are, in fact, the last group of sūtras ever to enter the sūtra section of the Kangyur, and are not present in some of the peripheral Kangyurs that may represent earlier compilations, such as the Phuktrak and Newark Batang Kangyurs.
As their colophons record, they were translated in the first decade of the 14th century at the monastery of Tharpa Ling (not far from Zhalu in Central Tibet), by a translator from Ceylon, Ānandaśrī, and the Tibetan translator Tharpa Lotsawa Nyima Gyaltsen Palzangpo, who was one of Butön's teachers.
The thirteen sūtras all have closely matching equivalents in the Pāli Canon, and—although they have at times been thought to be translations of Sarvastivādin texts in Sanskrit, as are many other Śrāvakayāna works in the Kangyur—it is almost certain that they were translated from Pāli, and are Theravāda texts, i.e. from the literature of the Theravāda school. Together with a translation of the third chapter of the Vimuttimagga that, despite being an extract of a treatise, is included for unknown reasons in the General Sūtra section (Toh 306), these are the only Pāli Theravāda works represented in the Kangyur (along with another translation by Ānandaśrī and Tharpa Lotsawa, a version of the Maitrīsūtra closely related to Toh 35 but found only in the Berlin, Lhasa, Narthang, and Peking Kangyurs).
Two of the thirteen texts, Toh 33 and 34, are the Theravāda equivalents of two of the ten Mūlasarvāstivādin Mahāsūtras, Toh 656/1061 and Toh 653/1062 respectively.
The thirteen works are sometimes referred to as Paritta texts, i.e. texts read aloud for protection, a status they hold in the Theravāda tradition; their association with auspiciousness may also partly explain why they were placed at the conclusion of the Perfection of Wisdom section.
For further details and analysis, see Skilling, Peter (1993), “Theravādin Literature in Tibetan Translation,” Journal of the Pāli Text Society, vol. 19, pp 69-201.