Photo of a seal used by Thomas Mifflin as the President of the Continental Congress in 1783, which is a derivative of the <a href="https://en.wikipedia.org/wiki/Great_Seal_of_the_United_States" class="extiw" title="w:Great Seal of the United States">Great Seal of the United States</a>.  It is possible the seal was used by all Presidents of the Congress, but the seal apparently did not pass over to the federal government in 1789.  This seal was on the envelope of a letter sent by Mifflin to the governor of Rhode Island on November 23, 1783, relating to the Treaty of Paris.  The letter is preserved in the state archives of Rhode Island, but the envelope is now missing.  The photo was taken in 1945 by Commodore Byron McCandless (a longtime naval officer who helped with the redesign of the president's flag, and father of <a href="https://en.wikipedia.org/wiki/Bruce_McCandless" class="extiw" title="w:Bruce McCandless">Bruce McCandless</a>) and sent in a May 29, 1945 letter to Captain James K. Vardaman (naval aide to President Truman), and is now archived in the Truman Library.
Commodore Byron McCandless (1881–1967).  Photo courtesy of the Harry S. Truman Library.
November 23, 1783 (seal), 1945 (photo)
Scanned from page 565 of <i>The Eagle and the Shield</i> by Richard Patterson and Richardson Dougall, 1978.