See http://www.gnu.org/manual/cvs/html_chapter/cvs_15.html
and http://www.gnu.org/manual/cvs/html_chapter/cvs_14.html
for how things are usually done.

The tricky issue is that the modules file also dictates
what is in core and that is not really tied to a particular
version so you still have issues trying to check out an
older release from cvs.