Lincoln auction at Sotheby's next Wednesday

[RJNorton]

(05-20-2016 10:34 AM)Angela Wrote:  The Gilder Lehrman Institute of American History is, for some reason, parting with one of the three existing Manuscript Copies of the Thirteenth amendment to the United States Constitution, signed and endorsed by Lincoln.

Does anyone know why Lincoln signed the Thirteenth Amendment? Did he know he was not required to do so? (I may be wrong, but I think it's the only amendment signed by a President.)