The article focuses on the critical analysis of sources of the late 16th century - “shert’” letters, that were discovered by the author in the Russian archive of ancient acts. The author discusses the origins of these documents and the circumstances of their discovery in the Archive’s funds. The first discovered source is the shert’ of Khan Kuchum given to Tsar Ivan the Terrible in 1571/72. Although this document was known in the first half of the 17th century, its original was considered lost. This paper presents a transcription of this shert’ letter, preserved in the collection of diplomatic correspondence. The second discovered source is “shert’ of Yermak” 1580 (1582). Different versions of this document are contained in the Esipov Chronicle, in the Pogodin Chronicle, as well as in a copy of the turn of the 18th - 19th century, as part of documents collected by A. F. Malinovsky. The last-mentioned version was published by V. I. Sergeev in 1976. This publication, however, has several inaccuracies. Comparison of publications and archival versions of sources showed, that over the years, researchers used defective editions, that had a bunch of incorrectness. In light of the fact, the need for further archival research aimed to discover the earlier existence of these valuable sources becomes apparent. The text of the documents is published in this article according to the simplified rules for publishing historical sources.