Chapman, Samuel, 1st Lieutenant, Company K, 15th Illinois Cavalry

M539: Index to Compiled Service Records of Volunteer Union Soldiers Who Served in Organizations from the State of Illinois

If you believe this is your man, his records should provide you the details of his service, You can procure these records through a service provided by these Message Boards, see:

http://history-sites.com/research/

PS, I could not find a pension record in his name or widows.

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Company "K" 15th Illinois Cavalry
This Company was formerly attached to the Thirty-sixth Regiment Illinois Volunteer Infantry, and known as Company "B" Dragoons, subsequently assigned as Company "K," Fifteenth Regiment Illinois Volunteer Cavalry

CHAPMAN, Samuel 1st Lt. Plato Sep 23, 1861 Resigned Apr 1, 1862

http://civilwar.illinoisgenweb.org/acm/cav015-k.html

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The companies that composed the Fifteenth Cavalry were independent companies, attached to Infantry Regiments, and acted as such.
http://civilwar.illinoisgenweb.org/history/c15cav.html

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See comment re Plano in:

http://www.aurorapubliclibrary.org/wp-content/uploads/WC19.pdf