I trust that letter is dated April 1864. What was the date and return address of the letter?
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The listing found on my web page may help. As you know I'm also trying to better understand the last year of the War. Please post or send additons and corrections.
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http://www.bourlandcivilwar.com/1864-65FrontierMilitaryUnits.htm
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Patti, prochette@Juno.com

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Quoting:
In a letter to Col T. Scott Anderson concerning the forming of Anderson's Regt. Tex. Cav., from Capt John P. Border, Border's Batln. Tex. Cav. wrote that he had 7 companies of cav. and 2 sections of artillery. When Anderson's Regiment left Camp Ford it reported taking two section of artillery with it.