Barbara -
I would be very interested to see the letter from your William Keller when you are able to post it. I've never seen any personal letters from my William Keller ancestor (William B. Keller), who may have been a cousin of some sort.
A second mystery about him (the first being the identities of his parents) is that after the war he was always referred to as Capt. Keller, even though, as my family has known for decades, he was a 1st Sgt who only became a 2nd Lt., and that was after Lee's surrender. It's a little embarassing, and may have be due to his inflated self-importance, but on the other hand he seems to have been a good soldier and was a responsible citizen after the war. I recently saw in Caldwell's History of Indiana Co., that his Regiment, 67th Pa, which had been captured almost entirely and the battle of 2nd Winchester, and the officers kept captive for long after the enlisted soldiers were released. Caldwell said that the companies were led by Sergeant until the officers were released. Maybe this is why W. B. Keller was called Captain. I hope to find out some time.