This is a tough question. +1 gmatbull.
My take:
On a book printed in 1955, there is a handwritten Quote( from treaty) with date 1956 & name Nelson.
Manuscript was not printed until 1968 and only 2 manuscripts were available before that
Chances are that Nelson saw this manuscript and wrote the quote himself sometime in 1956 but it looks like a forgery i.e Nelson did not write it in 1956 Since......
A. Nelson Mb is known to have used several different styles in writing his signature --> Nelson could have written it. No indication of forgery
B. Nelson Mb died not long after the publication of the printed edition of the treaty--> Nelson could have written it. As he might have had access to the manuscript. No indication of forgery
C. Nelson Mb was one of relatively few people who could still read Anglo-Saxon in the 1900s--> Nelson could have written it. As he knew how to read the manuscript. No indication of forgery
D. the printed edition of the treaty was based on only one of the two manuscripts, the other having
been known to very few scholars before 1968--> Nelson could have written it. As he might have had access to the manuscript. No indication of forgery
E. the quotation follows exactly the spelling of the printed edition, which contains errors introduced by the printer. ---> The Quote had a spelling mistake same as that found in the printed edition. So that means the quote was written after 1968 and not 1956.This is an Indication of forgery
E IMO