Buell was not really a cannoneer he was a private in the 20th Regiment, New York Cavalry.
After fist publishing his imaginative account in the National Tribune in 1890 Augustus Buell would write the book, The Cannoneer
based off his original National Tribune publication and letters like these that were written in responce.
On page 7 Major Clarke of Chamberlain's 185th New York writes Buell to insist it was Chamberlain's First Brigade that first received the white flag at Appomattox,
not the 188th New York in Gregory's Second Brigade (Though the Second Brigade was also under Chamberlain's command.)
He also insists it was Hiram Clark who was the last man killed in battle. Chamberlain had mentioned
Clark being killed in his official report
On page 13 an interesting tidbit from J. L. Pray on Warren being relieved of command,
"Gen. Chamberlain has since told me, in conversation at Gettysburg in July, 1888, that Col. Mike Sheridan had told him
the reason that no adjustment of the matter was ever made, was the proud spirt of both, and especially of Warren,
who would not allow any correction to be made, except through the "regular channels" of Court of Inquiry, etc."
Page 23 a Mr. Loomis of the 185th New York also writes to insist the last man killed was Hiram Clark
To look more into the story of the flag of truce see Appomattox
And for the last man killed see Surrender of Lee and his Army