I just bought this in an ebay auction. I compared characteristics with another definitely authentic item I found online and the script itself is that of Boston Corbett. Problem is, to my eye this appears to be printed even though the paper is yellowed and appears old. Maybe it's just the paper it is printed on. I can't tell. If so it would be a rather elaborate forgery. My cost was $199 with no other bidders.
Reference a Corbett autographed letter from Heritage Auctions dated July 13, 1878 just two weeks after mine.
http://historical.ha.com/c/item.zx?saleNo=6034&lotIdNo=31003
The pen used is completely different. The ink on mine is much thicker. As I said, it looks printed to me. Any comments or observations?