Seal of the Grand Duke of Lithuania <a href="https://en.wikipedia.org/wiki/Vytautas" class="extiw" title="w:Vytautas">Vytautas the Great</a>.
Marian Gumowski
Gumowski M. Pieczecie Ksiazat Litewskich // Ateneum Wilenskie. Z. 3—4, 1930. S. 722—723.