Dan Buck, a Cassidy historian, is quoted in the LA Times recently as having this to say: 

“Total horse pucky.  It doesn’t bear a great deal of relationship to Butch Cassidy’s real life, or Butch Cassidy’s life as we know it.”

The story apparently does not closely reflect substantiated events, a fact explained by the manuscript’s owners as Cassidy writing a fictionalized account of his life."

In the book business we see this every day.  Old books and other things generally are subject to grade inflation.  A miserable copy is a fair copy; the fair copy is a good copy, the good copy an outstanding one, the outstanding copy “the finest I have seen in 50 years in the trade.”  And then of course there is the first person account written 30 years after the writer’s death, the true black tulip, the one that defies memory and science.   “You have to see it to believe it.”

The book business is the awkward combination of emotion and logic.  Logic should always win and rarely does because where there are books there is often unjustified hope.  In this case I hope to hear the resolution of this case in the next year or so.  If the story is true the account will parade through the rooms at Sotheby’s or Christie's with all the pomp of Napoleon entering Notre Dame and then be sold for countless millions.  Or, the piece will become an interesting artifact and like Custer’s hair disappear but be whispered about in bedtime stories for generations to come.

LA Times article on this subject.