After kicking this around some more, I found the problem.  From the
0.76 ChangeLog:

1999-02-10 02:29:28  Lars Magne Ingebrigtsen  <larsi@gnus.org>

        * mail-source.el (mail-source-fetcher-alist): Remove apop.

        * nndoc.el (nndoc-type-alist): Remove MIME-digest.
        (nndoc-mime-digest-type-p): Removed.


When I ran the gnus-summary-read-document, nndoc-article-type came
back as mime-parts as opposed to mime-digest.

Why was nndoc-mime-digest-type-p removed?