I recently installed Spamassassin 2.64. In the spamassassin.org pages the tag MPART_ALT_DIFF is listed, however it isn't in the source code. It looks like it should be in "rules/50_scores.cf".
the spamassassin.org "Tests performed" page is currently based on the SA 3.0 code. It's been that way for several weeks now in prep for the 3.0 launch.
In general, you should never expect this page to be consistent with your version, particularly when the devels are getting ready for a new major release.
