Just as“only Nixon could go to China,” in the 1860s the prevailing political wisdom was that a slaveholder/slave state politician had to be the lead person in regard to a constitutional amendment to free the slaves. The 13th Amendment is one of the more thoroughly documented and studied pieces of legislation in American history, and the “claim” that Henderson authored it is impossible to refute and simple to support. With just a cursory review of my own files, here is a sampling of what is out there--

For a comprehensive account coming straight from the horse’s mouth, see John B. Henderson, “Emancipation and Impeachment,” The Century Illustrated Monthly Magazine, vol. LXXXV, Nov. 1912-April 1913, pp. 196-209

For a sampling of reports regarding John B. Henderson and the 13th Amendment as events were unfolding see--
New York Times, 9 April 1864
New York Times, 12 Jan. 1864
St. Louis Republican, 12 Jan. 1864
St. Joseph Herald, 10 Feb. 1865
St. Joseph Herald, 11 Feb. 1865
Louisiana (Mo.) Journal, 27 Jan. 1866
Congressional Globe, 38th Congress (4 March 1863-3 March 1865), 1st Session, p. 145

For a reference from Abraham Lincoln’s private secretaries, John G. Nicolay and John Hay, see
Abraham Lincoln: A History, X, New York, 1904, p. 75

For reference from the Clerk of the U.S. House of Representatives at the time the 13th Amendment was before Congress see--
Edward McPherson, The Political History of the Untied States of America During the Great Rebellion (New York, 1864), p. 218, 255

For detailed accounts regarding Henderson’s interaction with Abraham Lincoln on the matter see--
St. Louis Globe-Democrat, 18 Jan. 1888, p. 6
Walter B. Stevens, A Reporters Lincoln,” Lincoln: University of Nebraska Press (see section entitled “John B. Henderson--Emancipation: Mr. Lincoln’s Connection with the Bill to Compensate Loyal Slaveholders” at pp. 170-173). This is a 1998 reprint of a book originally published in 1916. Stevens lived from 1848-1939, and was the Washington D.C. bureau chief for the St. Louis Globe-Democrat.

For a sampling of reports regarding Henderson’s evolving views on the issue of slavery and Emancipation see--
Journal of the Missouri House, 1849, p. 82
Weekly Jefferson Inquirer, 21 April 1849
Louisiana Democratic Banner, 30 July 1849
St. Louis Republican, 24 July 1858
St. Louis Republican, 12 April 1860
St. Louis Republican, 1 April 1862
Louisiana (Mo.) Journal, 10 April 1862
Louisiana Journal, 14 August 1862
St. Louis Republican, 26 July 1862
Louisiana Journal, 7 August 1862
Louisiana Journal, 15 Jan. 1863
Journal of the Missouri State Convention, June 1862, (St. Louis 1862) pp. 19-20
Proceedings of the Missouri State Convention June 1862, (St. Louis 1862) pp. 98, 99, 104
Congressional Globe, 37th Congress (4 March 1861-3 March 1863), 2nd Session, p. 1393
Congressional Globe, 37th Congress, 3rd Session, pp. 52, 138, 351-357, 587,
Walter H. Ryle, Missouri: Union or Secession, Nashville 1931, p. 140
Walter B. Stevens, “Lincoln and Missouri,” Missouri Historical Review, January 1916, p. 83
William Hyde and Howard L. Conard,eds., Encyclopedia of the History of St. Louis, (St. Louis, 1899), p. 1016

For a few 13th Amendment references from contemporary Henderson historians see--
John F. Kennedy, Profiles in Courage, New York 1956, p. 167
Arthur H. Mattingly, “Senator John Brooks Henderson, United States Senator From Missouri,” Ph.D. dissertation, Kansas State University, 1971
Ralph J. Roske, “The Seven Martyrs?” American Historical Review 64 (January 1959) 323-30
Hans L. Trefousse, Radical Republicans, New York 1969, p. 298
Bob L’Aloge, Emancipation and Henderson, Las Cruces 1990, 49-54, 84-87, 93-98