It would be in a multi volume set of books called:
"The Adjutant Generals Letter Book"
If I remember correctly it would be the outgoing correspondance of the Adjutant Generals.
What you would be looking for would be around Volume 21...22....or 23....somewhere in that area.
The letter you want will be on Page 153 if you can figure out which volume.
They don't want you handling the original letters so they typed the letters into these volumes.
I happened to have a copy of the page you want and scanned it and uploaded it here:
http://img22.imageshack.us/i/cobbmilitia.jpg/
That is exactly what you will see in the book.
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