Apparently a rather new tool for comparison of old versions of a page; discovered in hyperlinks in the output from New Recent Changes.
Example: c2.com 
Rather less usefully, c2.com
History diff hack:
Compare any "older page version number" to any "newer page version number" (version numbers refer to versions in c2.com ).
http://c2.com/cgi/histDiff.cgi?page=[X]&old=[Y]&new=[Z]
X = name of page Y = older version number Z = newer version number
Example: c2.com
(This breaks time, and by whom i.e.: Revision 88 made 37 years ago by)
Numbers can also be reversed: c2.com
Q: Hist Diff output seems to be eliding all numbers. Is this a bug?
A: Yes, this unlikely bug was fixed 2005/02/26.
See also: Hidden Features, New Recent Changes
See original on c2.com