I do not know but assume it's a National Parks Service document and most likely found somewhere in the National Archives.

This is the first time I’ve come across this reference. It’s important as the only document found referencing his Confederate service.

Suggest you query the Prisons Message Board re this document
http://www.andersonvillepowcamp.com/index.php?page=directory&rec=22752

http://www.history-sites.net/cgi-bin/bbs62x/cwpmb/webbbs_config.pl

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See also:

Posted: Saturday, September 27, 2014 1:37 pm
BY LORAINE ANDERSON Local columnist

I awake early enough to see a dark sky studded with bright stars. They spark a deep sense of connection to something so much larger than myself. I stand on my back porch, looking up.

Four Civil War veterans from my hometown come to mind: Drummer Wallace E.H. Powelson and three others who may be brothers — Nicholas, John and Walter C. Schermerhorn.

All lived, slept and survived under southern skies for the three to six years they served in the Civil War.
All settled in the then-wilderness of Mayville in Michigan’s Thumb just before, or soon after, the Civil War.
They are among the names of 126 Civil War soldiers I’m researching this summer for my hometown’s museum of history and genealogy. I have learned so much from them about the war, their times and Mayville’s history.

Powelson and John Schermerhorn, a bugler in the Seventh Michigan Cavalry, spent time in Andersonville prison. Both were captured in late 1863 — John in October at Culpeper Courthouse, Virginia, and Wallace near Knoxville, Tennessee, in December. John Schermerhorn died at Andersonville in May 1864 and is buried there.

Powelson was 18 when he enlisted in 1862 in the 23rd Michigan Infantry. According to federal military records, he may have been a “galvanized” Union soldier who changed sides to escape the horrors of the notorious Confederate prison, then fled his unit and found his way back to Union lines and his regiment.

“Galvanize” is an apt word.

The brotherhood of John, Nicholas and Walter Schermerhorn is yet a mystery and a hunch.

http://www.record-eagle.com/news/lifestyles/loraine-anderson-early-morn-mysteries-and-hunches/article_0a6a6f08-6042-5522-808a-e81da79102d7.html

…….

Powelson, Wallace E. H., East Saginaw. Enlisted in company B, Twenty- third Infantry, as Musician, Aug. 4, 1862, at East Saginaw, for 3 years, age 18. Mustered Sept. 11, 1862. Taken prisoner at Buffalo Creek, Tenn., Dec. 11, 1863. Escaped from prison at Augusta, Ga. Returned to regiment May 16…

http://babel.hathitrust.org/cgi/pt/search?q1=Powelson;id=mdp.39015020464478;view=1up;seq=13;start=1;sz=10;page=search;orient=0